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Machine A produces bolts at a uniform rate of 120 every 40

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Machine A produces bolts at a uniform rate of 120 every 40 [#permalink] New post 07 Dec 2012, 03:03
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Machine A produces bolts at a uniform rate of 120 every 40 seconds, and Machine B produces bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 200 bolts?

(A) 22
(B) 25
(C) 28
(D) 32
(E) 56
[Reveal] Spoiler: OA
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Re: Machine A produces bolts at a uniform rate of 120 every 40 [#permalink] New post 07 Dec 2012, 03:05
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Walkabout wrote:
Machine A produces bolts at a uniform rate of 120 every 40 seconds, and Machine B produces bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 200 bolts?

(A) 22
(B) 25
(C) 28
(D) 32
(E) 56


Machine A produces 120/40=3 bolts per second;
Machine B produces 100/20=5 bolts per second.

Running simultaneously, they produce 3+5=8 bolts per second, thus it will take them 200/8=25 seconds to produce 200 bolts.

Answer: B.
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Machine A produces bolts at a uniform rate of 120 [#permalink] New post 12 Nov 2013, 07:49
Machine A produces bolts at a uniform rate of 120 every 40 seconds, and Machine B produces bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 200 bolts?

(A) 22
(B) 25
(C) 28
(D) 32
(E) 56


why cant i do it this way:
take 1/time taken for each machine to produce 20 bolts: thats (1/60) for A plus (1/100) for B. Then I add them up and take the reciprocal to get total time taken. thats 37.45. so its 37.45 bolts every 20 secs. so that 106.8 seconds for 200 bolts.

whats wrong with what I did?
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Re: Machine A produces bolts at a uniform rate of 120 [#permalink] New post 12 Nov 2013, 08:06
Machine A produces 120 bolts every 40 seconds.
How many bolts does Machine A produce every second?
Machine A produces 120/40 = 3 bolts every second.

Machine B produces 100 bolts every 20 seconds.
How many bolts does Machine B produce every second?
Machine B produces 100/20 = 5 bolts every second.

Together, Machine A and Machine B produces 3+5 = 8 bolts every second.

8 bolts / 1 sec = 200 bolts / x sec

Cross multiply, and get 200 = 8x; so, 25 = x.
So it takes 25 seconds to produce 200 bolts, when Machine A and Machine B work together.




Quote:
why cant i do it this way:
take 1/time taken for each machine to produce 20 bolts: thats (1/60) for A plus (1/100) for B. Then I add them up and take the reciprocal to get total time taken. thats 37.45. so its 37.45 bolts every 20 secs. so that 106.8 seconds for 200 bolts.


Unfortunately, I'm having trouble following your logic. It looks like you try to find how much time it takes to produce 20 bolts, but then later try to figure out how many bolts are produced every 20 seconds?
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Re: Machine A produces bolts at a uniform rate of 120 [#permalink] New post 12 Nov 2013, 08:23
I apply the simple formula: 1/(time taken by machine A) + 1/(time taken by machine B) = 1/(total time taken by both machines, together. this is now the rate (together). to get total time taken, inverse it. but i see what i did. i started adding the total bolts, thinking thats the time. i see my error, thank you. but the formula mentioned above (in my post) is correct, right?
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Re: Machine A produces bolts at a uniform rate of 120 [#permalink] New post 12 Nov 2013, 09:12
Expert's post
usre123 wrote:
Machine A produces bolts at a uniform rate of 120 every 40 seconds, and Machine B produces bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 200 bolts?

(A) 22
(B) 25
(C) 28
(D) 32
(E) 56


why cant i do it this way:
take 1/time taken for each machine to produce 20 bolts: thats (1/60) for A plus (1/100) for B. Then I add them up and take the reciprocal to get total time taken. thats 37.45. so its 37.45 bolts every 20 secs. so that 106.8 seconds for 200 bolts.

whats wrong with what I did?


Merging similar topics. Please refer to the solution above.

Also, please search before posting.
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COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Machine A produces bolts at a uniform rate of 120 every 40 [#permalink] New post 11 Sep 2014, 05:35
Walkabout wrote:
Machine A produces bolts at a uniform rate of 120 every 40 seconds, and Machine B produces bolts at a uniform rate of 100 every 20 seconds. If the two machines run simultaneously, how many seconds will it take for them to produce a total of 200 bolts?

(A) 22
(B) 25
(C) 28
(D) 32
(E) 56













Machine A .. 120 in 40 seconds then 3 /sec
machine B .. 100 in 20 seconds then 5/sec

in one sec both can produce 8.

so for 200 , 200/8 = 25
Re: Machine A produces bolts at a uniform rate of 120 every 40   [#permalink] 11 Sep 2014, 05:35
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