How far upstream can a motorboat travel at 8 mihr and still

How far upstream can a motorboat travel at 8 mi/hr and still return downstream at 12 mi/hr in a total time of 4 hr?

Solution

How far upstream can a motorboat travel at 8 mi/hr and still return downstream at 12 mi/hr in a total time of 4 hr?
let the time taken upstream be x
the down stream time will be 4-x hrs.
speed upstream = 8mph
distance upstream = 8*x
distance down stream = 12*(4-x)
8x= 48-12x
16x=48
x=3 hour---------------------------------- 24 miles
down stream time = 1hour