Acyclist rode 1 mile at full speed in 3 minutes with the wind and returned in 4 minutes against the wind, again pedaling at full speed. find his speed without the wind. ("challenge" question)

So 1 mile per 3 minute or
1 times 20=20
3 times 20=60

20 miles in 60 minutes or
20 miles per hour

20 mph with wind

4 mins agains the wind
multiply 15
15 miles per hour agains the wind

so d=st

distances are the same
d=st
20mph with the wind
20=wind+real speed

15mph agains the wind
15=real speed-wind

20=w+rs
15=-w+rs
add together
35=2(rs)
divide 2
real speed=17.5