Question 33336
Upstream data:
distance=x miles;  rate = 3mi/hr;   time= x/3 hrs.
Downstream data:
distance=x miles;  rate = 5mi/hr;   time= x/5 hrs
EQUATION:
time up + time down = 48 min= 48/60 hrs.
x/3 + x/5 = (4/5) hr. 
Multiply thru by 15 to get:
5x+3x=12
9x=12
x=4/3 mi = 1 1/3 miles
Cheers,
Stan H.