Question 192044
Let time it takes to go up the river be u and time it takes to go down be w.
u+w=5
u=5-w
.
d=st d=distance, s=speed, t=time
st=st since the distances are equal. 
8u=12w  
8(5-w)=12w substitute 5-w for u.
40-8w=12w
20w=40
w=2 hrs 
u=3 hrs
8*2=16 mi distance at which you should turn back.
7 AM + 3 hrs= 10 AM time you should turn back.
.
Ed