A concave mirror forms a real image that is 4 times as tall as the object.

Step 1. The signs. $v = -80\ \text{cm}$ (real, in front) and $m = -4$ (real, inverted, four times taller).

Step 2. The object. $u = -\dfrac{v}{m} = -\dfrac{-80\ \text{cm}}{-4} = -20\ \text{cm}$.

Step 3. The focal length. $\dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u} = -\dfrac{1}{80\ \text{cm}} - \dfrac{1}{20\ \text{cm}} = -\dfrac{1 + 4}{80\ \text{cm}} = -\dfrac{1}{16\ \text{cm}}$, so $f = -16\ \text{cm}$.