Chris' average speed is 3.5mph:
Lets take Chris' brother speed = B
then Chris speed = (B+ 1) mph-------given in the question
and:
The average speed for covering 18 miles= chris' average speed + his brother's
average speed = (B+ 1) + B
since Speed = Distance covered / time taken
then average speed for 18 miles can also be found by: 18 miles/3 hours = 6 miles per hour
then we have; (B+ 1) + B = 6mph
(B +1) = (6-B)
B+B = 6-1=5
2B=5
B=5/2=2.5 mph (this is Chris' brother average speed)
Hence Chris' average speed is = B+1= 2.5+1= 3.5mph
