Originally Posted by

**nzmathman** $\displaystyle distance = velocity \times time$

Let t = time taken to travel downstream.

Let v = the velocity of the boat in still water.

using $\displaystyle d = vt$,

$\displaystyle 5 = (v - 4) \times (t + 20)$

$\displaystyle 5 = (v + 4) \times t$

We want 'v' on its own so we can solve, so divide the 'v parts' across to the other side.

$\displaystyle \frac{5}{v-4} = t + 20$

$\displaystyle \frac{5}{v+4} = t$

$\displaystyle \frac{5}{v - 4} = \frac{5}{v+4} + 20$

Multiply whole expression by $\displaystyle (v + 4)(v - 4)$

$\displaystyle 5(v + 4) = 5(v - 4) + 20(v + 4)(v - 4)$

Now simplify and solve the quadratic formed.

Don't hesitate to ask for further help...