Ok, so if he walks 4 miles per 64 minutes, or 4 miles/64 min which will reduce to 1 mile/ 16 min, then the numbers of miles he walks in a given number of minutes would be
1/16 * number of minutes
The question states we represent miles with y and number of minutes with x, so...
y = 1/16x
So let's check it...
How many miles (y) in 64 minutes (x)
y = 1/16x
y = 1/16 * 64
y = 64/16 = 4 miles
Yep! It checks!