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Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 06:59
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 21:08
@ Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. Simply take the LCM of 40 and 45  you get 360. This is the time they will take to meet for the first time. In this time, Michael would have covered 360/40 = 9 laps. Answer (B) The reason for this: Judging from the question, we know that Michael and Donovan are running in the same direction. When will Michael overtake Donovan? Michael will always be a bit ahead of Donovan and will keep increasing this distance between them till the time he completes one full lap more than Donovan. That is when they will both again be at the starting point together. The time that should have passed at this point should be a multiple of both 40 and 45 so that they are at the starting point. So we find the LCM of 40 and 45 which is 360. In this time, Donovan completes 360/45 = 8 laps and Michael completes 360/40 = 9 laps.
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 07:36
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. As per options given, Assuming that the two runners are running along the track in the SAME DIRECTION Speed of Michael = Distance/Time = 400/40 = 10 m/Sec Speed of Donovan = Distance/Time = 400/45 = 80/9 m/Sec Relative Speed for the two objects moving in the same direction = Difference between their speed Note: Relative Speed = The rate at which the distance between the two moving objects increases or decreases In same Direction, The Distance Increases at lower rate, and the Rate = S1  S2 In Opposite Direction, The Distance Increases at Higher rate, and the Rate = S1 + S2Hence Relative Speed of Michael and Donovan = 10  (80/9) = 10/9 m/sec Relative Distance to be covered by Michael to pass Donovan = Length of complete Track = 400 mt Time Taken by Michael to pass Donovan = Relative Distance / Relative Speed = 400 / (10/9) = 360 Secs No. of Laps completed by Michael in 360 Secs = Total Time to pass Donovan / Time of Michael per Lap = 360/40 = 9 Answer: Option
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 07:51
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. ALTERNATE METHOD: Since Time taken by Michael to cover track < Time taken by Donovan to cover track therefore, Speed of Michael > Speed of Donovan Now, Let's say that Michael passes Donovan in 't seconds' time Then Michael will have to cover the distance of 400 mt more than Donovan's distance in order to pass him running in the same direction.Therefore if Distance covered by Donovan = D mt then Distance covered by Michael = (D+400) mtSpeed of Michael = Distance/Time = 400/40 = 10 m/Sec Speed of Donovan = Distance/Time = 400/45 = 80/9 m/Sec For Constant Time S1 / S2 = D1 / D210 / (80/9) = (D+400) / D i.e. 9 D = 8 D + 3200 i.e. D = 3200 mt. Laps Completed by Michael = Total Distance Covered by Michael / Distance per Lap = (3200+400) / 400 = 9 Answer: Option
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 07:52
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. Speed of D=400/45=8.88(approx)m/s Speed of M=400/40=10m/s Rel speed =1.2m/s So M gets a lead of 1.2 m in one second so in order to get a lead of 400m he will take=400/1.2=333.33 sec and it equivalents to 333.33/40=8.33 rounds and hence ans is B.



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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 19:23
Hi All, Since we have the "lap speeds" of each of the two runners, the length of the lap is IRRELEVANT. The "math" behind this question is the same as a "Chase Down"/Combined Rate question. Donovan runs each lap in 45 seconds. Michael runs each lap in 40 seconds. The question asks how long it will take Michael to "pass" (re: "lap") Donovan. Since the difference in their two times is 4540 = 5 seconds, Michael will "catch up" 5 seconds with every lap. Donovan spends 45 seconds per lap, so Michael will need 45/5 = 9 laps to catch Donovan. Final Answer: GMAT assassins aren't born, they're made, Rich
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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16 Feb 2015, 20:41
jmo: the easiest way to nail down this question s to plug the number in for 8: 400*8 = 3200, both 3200/45 and 3200/40 are not integer, kill for 9: 400*9 = 3600, both 3600/45 and 3600/40 are integer, so they overlap. > Correct answer is B in 30 seconds.



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Re: Donovan and Michael are racing around a circular 400meter track. If D
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17 Feb 2015, 01:24
Answer = B. 9 LCM of 40 & 45 = 360 360 is the point wherein both will "meet" after starting up of the race. Laps required by Michael \(= \frac{360}{40} = 9\)
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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17 Feb 2015, 02:17
[quote="Bunuel"]Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution.[/quote
One way of approaching this question is by Relative speed method 1. Speed/ Rate of Donovan = Distance/ time => 400/45 =>80/9 2. Speed/ Rate of Michael = Distance/ time => 400/40 => 10
Relative Speed between them = 10  80/9 => 10/9 (We subtract the Rates if moving in the same direction and add the rates if moving in the opposite direction)
In order to pass Donovan Distance to be covered = 400, Relative Rate = 10/9
Total Time taken by Micheal to surpass Donovan = Distance / rate => 400*9/10 => 3600/10 => 360
No. of laps taken by Michael = Total time / Michael's rate => 360/40 => 9
Hence correct answer is 9 Laps.



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Re: Donovan and Michael are racing around a circular 400meter track. If D
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22 Feb 2015, 11:25
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. VERITAS PREP OFFICIAL SOLUTIONSolution: B. This problem can be tricky with algebra, but with logic it’s much simpler. On each lap that Donovan runs, Michael runs an additional 1/8 of a lap with the additional 5 seconds. In order to get a full lap ahead, Michael will need Donovan to have run 8 laps. The “trick” comes here – Donovan will have run 8 laps, but Michael will have completed his ninth. So the correct answer is B – when Donovan completes his 8th lap, Michael will be completing his ninth.
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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15 Feb 2016, 16:24
M's rate is 1/40 lap per second D's rate is 1/45 lap per second (1/40)/(1/45)=9/8 M runs 9 laps for every 8 laps D runs



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Re: Donovan and Michael are racing around a circular 400meter track. If D
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21 Mar 2017, 06:08
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12 We are given that Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds. Thus, their respective speeds are 400/45 = 80/9 meters per second and 400/40 = 10 meters per second. If Michael passes Donovan t seconds after they start running, he will have traveled exactly 400 meters more than Donovan (because 400 meters is equal to one lap). Since distance = rate x time, we can say that in t seconds, Donovan covers a distance of 80t/9 meters and Michael covers a distance of 10t meters. Since the difference between these distances has to equal 400, we have the following: 10t  80t/9 = 400 90t/9  80t/9 = 400 10t/9 = 400 10t = 3600 t = 360 seconds In 360 seconds, Michael completes 360/40 = 9 laps. Answer: B
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Re: Donovan and Michael are racing around a circular 400meter track. If D
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23 Mar 2017, 12:44
Michael makes a lap in 40 sec. He makes x laps. Time spent on road = 40x When Michael reaches Donovan, Donovan has made exactly 1 lap less. It takes him 45 sec to make a lap. Total time spent on road = 45(x1) 40x=45(x1) 5x=45 x=9 laps (B)



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Re: Donovan and Michael are racing around a circular 400meter track. If D
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03 Apr 2018, 10:04
Bunuel wrote: Donovan and Michael are racing around a circular 400meter track. If Donovan runs each lap in 45 seconds and Michael runs each lap in 40 seconds, how many laps will Michael have to complete in order to pass Donovan, assuming they start at the same time?
A. 8 B. 9 C. 10 D. 11 E. 12
Kudos for a correct solution. If Michael passes Donovan, then Donovan completes ONE LAP LESS than Michael completes IN THE SAME AMOUNT OF TIME. So, if x = # of laps Michael completes, then.... x  1 = # of laps Donovan completes. So, we can write the following WORD EQUATION: (time for Michael to complete x laps) = (time for Donovan to complete x  1 laps) It takes 40 seconds for Michael to complete EACH lap, and it takes 45 seconds for Donovan to complete EACH lap. So, we get: (40)(x) = (45)(x  1) Expand: 40x = 45x  45 Solve to get x = 9 So, Michael must complete 9 laps Answer: B Cheers, Brent
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