Question

The heights of three lamp-posts L1, L2 and L3 are proportional taken in the same order. If L1 is 10m tall and L3 is 90m, how tall is L2?

• A.
  60 m
• B.
  30 m
• C.
  20 m
• D.
  50 m

Answer 1

The correct option is B

30 m
Let the heights of L1, L2 and L3 be x ,y and z metres respectively.

Given
x = 10 m
z = 90 m
We need to find y
Also, it is known that they are proportional.

$\Rightarrow$ x : y :: y : z
$\Rightarrow$ $\frac{x}{y}=\frac{y}{z}$
$\Rightarrow$ $\frac{10}{y}=\frac{y}{90}$
$\Rightarrow$ $900=y\times y$
$\Rightarrow$ $30\times 30=y\times y$
$\Rightarrow$ $y=30m$
$\therefore$ The height of L2 is 30 m