ayl989 wrote:

If a bus took 4 hours to get from town a to town b, what was avg speed of bus for the trip?

1. in first 2 hours bus covered 100 miles.

2. avg speed of bus for first half of the distance was twice it's speed for the second half.

Let me know what you guys think, OA to follow.

TIA.

I think the asnwer is (C) - let me explain

dividing the trip into 2 parts where x = total road and y = speed

first part
100 miles = distance , 2y = speed , 100/2y = time

second part
x-100 miles = distance , y = speed , x-100/y = time

Statement 1
100/2y = 2

100 = 4y

y = 25

but we don't know what x is - and we can't find the average for the whole trip.

Insufficient

Statement 2
(100/2y) + (x-100)/y = 4

100/2y + (x-100)/25 = 4

50/y + (x-100)/25 = 4

we are still left with x,y

Insufficient

Statements 1&2
Only when we use statement 1 to find y and statement 2 to find x

we can calculate the average speed for the trip

50/

y + (x-100)/25 = 4

50/

25 + (x-100)/25 = 4

2*25 + (x-100) = 100

x = 150

the average for the trip is

(50*2+25*2)/4 = 37.5 mph

Sufficient - the answer is (C)