Question 5390
It is a question of average Speed. It has to be solved in three parts.

1.  Going from A to B     ---  2.  Going from B to A    --- 3.  Average Speed

1. Going from A to B

Distance = d1 = x miles
Speed = s1 = 10 miles per hour
Time =  t1 =  Distance/ Speed  =  x/10

2.   Going from B to A

Distance = d2 = x miles
Speed = s2 = 8 miles per hour
Time =  t2 =  Distance/ Speed  =  x/8

3.   Average Speed =  Total Distance/Total Time

Total Distance = x + x = 2x
Total Time = x/10 + x/8 = x (1/10 + 1/8) = x ( 4 + 5)/40 = 9x/40

Speed = 2x/(9x/40) = (2x * 40)/9x = 80x/9x = 80/9 = 8.9   

HOPE THIS IS CLEAR TO YOU.
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