Bunuel wrote:

The current in a river is 4 mph. A boat can travel 20 mph in still water. How far up the river can the boat travel if the round trip is to take 10 hours?

A. 69 miles

B. 88 miles

C. 96 miles

D. 100 miles

E. 112 miles

another way to solve the problem would be to think a little logically here.

We know that the distance upstream and the distance downstream is the same. therefore, upstream=downstream (eq1)

Also, if total time is 10, then assuming Upstream as T, Downstream will be 10-T. (Not T-10 since 10 is the total)

Using R*T=D formula we then get,

Upstream= 16*T=D (eq2)

Downstream= 24*(10-T)=D (eq3)

Now Using eq1 we get eq2=eq3. Solving that,

16t=24(10-t)

16t=240-24t

40t=240

t=6

substituting t=6 in the Upstream equation eq2 we get 96.

To verify, we can also substitute in eq3 which should also give 96.

Hope it helps.