Abird leaves its nest and travels 18 miles per hour downwind for x hours. on the return trip, the bird travels 3 miles per hour slower and has 5 miles left after x hours.

d = distance down wind d - 5 = distance back time = distance ÷ speed x = d/18 x = (d - 5)/(15) 
so d/18 = (d - 5)/15. Cross multiply 18d - 120 = 15d = 40-120 = -5d 24 = d so the whole trip assuming the bird gets back to the starting point is 48 miles..