Question 10618
Let v be speed of current
Upstream the speed of the boat is 8-v
Since boat travels 7 miles upstream it does it in time 7/(8-v)

Downstream the speed of the boat is 8+v
It travels 9 miles in time 9/(8+v)

 These times are equal

 7/(8-v) = 9/(8+v)

 Solving for v gives v=1 miles per hour