Question 701699
Fundamental relationship is speed multiplied by time equals distance, r*t=D
Let r equal rower's speed if in still water.
Let D equal the one way distance.


Use this format if building a table of data....
Direction: Speed-----------Time---------Distance
Downstream: r+2.5 ---------1.5  ---------D=(r+2.5)(1.5)
Upstream:  r-2.5 ----------4.5  ---------D=(r-2.5)(4.5)


Up and downstream were the same distance, just different rates, so equate the expressions for D:


{{{(r+2.5)*1.5=(r-2.5)*4.5}}}
Divide both sides by 1.5,
{{{r+2.5=3*(r-2.5)}}}
{{{r+2.5=3*r-7.5}}}
{{{2.5+7.5=2r}}}
r=5

In still water, the rower's speed is 5 miles per hour.  
You can finish by choosing either formula for D to compute the one way distance.