A homing device left its home to deliver a message. On takeoff, the device encountered a tailwind of 11 mph and made the delivery in 15 minutes. The return trip into the wind took 30 minutes. How fast could the device fly in still air?

let the speed be x miles/hr

on takeoff:

d = (x+11)*1/4

on return:

d = (x-11)*1/2

=> (x+11)/4 = (x-11)/2

=> 2x+22 = 4x-44

=> x = 33 miles/hr