This is a proportion problem. 2 is to 25 minutes as x minutes is to 137.5 minutes ( I assume you meant minutes). 
x= minutes walked in 137.5 minutes
2/25 = x/137.5
cross multiply 
2*137.5 = 25*x
275 = 25x
divide both sides by 25
11 = x
He can walk 11 miles in 137.5 minutes. 
Hope this helps! :)