A cyclist 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