# Bob is falling down from a height. If he falls x^2+5x m in x seconds, how many seconds does it take him to fall 500 m?

Mar 20, 2016

It is 20s.

#### Explanation:

We need to solve the distance formula for
${x}^{2} + 5 x = 500$.

This is a quadratic:
${x}^{2} + 5 x - 500 = 0$

We can factorise this as:
$\left(x + 25\right) \cdot \left(x - 20\right) = 0$

Hence $x = 20 s$