they were supposed to return in 3hrs, so down and up the stream is 3hrs total for the whole trip
the distance downstream and upstream is the same
[tex]\bf \begin{array}{lccclll}
&distance&rate&time\\
&-----&-----&-----\\
\textit{downstream}&d&20&t\\
\textit{upstream}&d&16&3-t
\end{array}\\\\
-----------------------------\\\\
\begin{cases}
d=20t\\
d=16(3-t)\\
----------\\
20t=16(3-t)
\end{cases}\implies 20t=48-16t\implies 36t=48
\\\\\\
t=\cfrac{48}{36}\implies t=\cfrac{4}{3}\impliedby \textit{1hr and 20mins}
\\\\\\
\textit{what's "d"?}\qquad d=20t\implies d=20\cdot \cfrac{4}{3}\implies d=26\frac{2}{3}\ miles[/tex]
