Let the initial speed of the boat be v.
Adding the times for each leg, we get:
[tex]\frac{16}{v}+\frac{12}{v+4}=3[/tex]
This equation results in the following quadratic:
[tex]3v^{2}-16v-64=0[/tex]
which factorizes to: (3v + 8)(v - 8) = 0
The positive solution is 8.
The answer is 8 mph.
