Question

How did the 16 come about (comments)?

Answer 1

\(V_{bg}\) is the velocity of the boat wrt ground
\(V_{rg}\) is the velocity of the river wrt ground
\(V_{br}\) is the velocity of the boat wrt river.

\(V_{br}^2=V_{bg}^2+V_{rg}^2\)

\(V_{bg}=\frac{140}{20},\quad V_{rg}=4\)

from \(V_{rg}^2\) you get 16

Answer 2

Thanks

Answer 3

you're welcome :)