Jack traveled the first 75% of an 80-mile trip at 45 miles per hour an

Question

Jack traveled the first 75% of an 80-mile trip at 45 miles per hour and the remainder at 30 miles per hour. What was Jack’s average speed for the entire 80-mile trip, in miles per hour?

(A) 37.5
(B) 38.25
(C) 40
(D) 41.25
(E) 42.5

rajatchopra1994 · Answer

Explanation:

Average speed = total distance/total time

we know the total distance is 80 miles
75% distance covered with 45 miles per hour i.e. 80x0.75 = 60 miles
time = 60/45 = 1.333

Rest distance i.e. 20 miles at 30 miles per hour
time = 20/30 = 0.666

Average speed = 80 / (1.333 + 0.666) = 40

IMO-C

mohammadfaraaz123 · Answer

Average Speed = Total Distance/ Total Time

Time = Distance/Speed

Distance 1  = 80 *3/4
 = 60 miles

Speed 1 = 45 m/h

Time 1 = 60/45 = 4/3hrs

Remaining Distance  = 80-60 = 20 miles

Speed 2 = 30 m/h

Time 2 = 20/30 = 2/3hrs

Average Speed = Total Distance / Total time

= 80/ (4/3 + 2/3)

= 80/2 
 =40 m/h

Answer option C

MathRevolution · Answer

First 75% of an 80-mile trip at 45 miles per hour:  \frac{75}{100} * 80 = 60  miles distance and speed = 45 miles per hour

Time taken:  \frac{60}{45} = \frac{4}{3}  hours

Remainder distance: 80 - 60 = 20
Speed = 30 miles per hour

Time taken:  \frac{20}{30} = \frac{2}{3}  hours

Total distance: 60 + 20 = 80

Total time:  \frac{4}{3} + \frac{2}{3} = \frac{6}{3} = 2

Average speed:  \frac{Total distance }{ Total time}  =  \frac{80 }{ 2}  = 40 miles per hour

Answer C

harjas2222 · Answer

will 2xy/ x+y not work here?

Bunuel · Answer

The formula 2s1*s2/(s1 + s2), where s1 and s2 are speeds, gives the average speed only when equal distances are covered at each speed. In the question at hand, the distances are not equal, so it does not apply.

rajdeep212003 · Answer

I hope this will be helpful

Jack traveled the first 75% of an 80-mile trip at 45 miles per hour an

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