A convex lens forms a real image three times the size of the object on a screen. If the focal length is 20 cm, the object distance is

Real image, $m=-3=v/u$ (using $m=v/u$ sign-aware: $v=-3u$ with lens sign convention $u<0$). Lens formula $\frac1v-\frac1u=\frac1f$. With $|m|=3$: $v=3|u|$. $\frac{1}{3|u|}+\frac{1}{|u|}=\frac{1}{20}\Rightarrow\frac{4}{3|u|}=\frac1{20}\Rightarrow|u|=80/3\approx26.7$ cm.