C

took just 40 second by backsolving...

total distance 400 miles..

start with B , let carB speed = 30

time taken = 400/30=13.33

we know car A speed with 10 greater than B so A = 40

Time taken = 400/40=10 ( time difference greater than 2 hour wrong)

try C , Car B speed 40 , time taken 10 hours

then Car A speed 50 , time taken 8 hours ...which is 2 hours less....

so ...ans C

joemama142000 wrote:

The time it took car A to travel 400 miles was 2 hours less than the time it took car B to travel the same distance. If car Aâ€™s average speed was 10 miles per hour greater than that of car B, what was car Bâ€™s average speed, in miles per hour?

(A) 20

(B) 30

(C) 40

(D) 50

(E) 80