Answer
$V_{r} = 1$ mph
Work Step by Step
$V_{p}$ = speed of Pamela
$V_{r}$ = river speed
Time to go downstream
$T_{1} = \frac{15}{V_{p} + V_{r}}$
$V_{p} + V_{r} = \frac{15}{3} = 5$ mph
Time to go upstream
$T_{1} = \frac{15}{V_{p} - V_{r}}$
$V_{p} - V_{r} = \frac{15}{5} = 3$ mph
$V_{p} + V_{r} = 5$
$V_{p} - V_{r} = 3$
Add the two equations
$2V_{p} =8$
$V_{p} = 4$
$V_{r} = 5-4=1$ mph