An electric pole casts a shadow of length 20 m at a time when a tree 6 m high casts a shadow of length 8 m. Find the height of the pole.

The correct answer is 15

Let $x$ be height of the electric pole.

Shadow of electric pole or tree at a certain time is proportional to its original length.
⇒ $x$ : 20 :: 6 : 8

⇒ $\dfrac{x}{20}$ = $\dfrac{6}{8}$

⇒ $x$ = $\dfrac{20 × 6}{8}$ = $15 m$

Hence, the height of the pole is $15m$