Question

A motor bike covers $432km$ using $36$ litres of petrol. How much distance would it cover if it uses $25$ litres of petrol?

Answer 1

Suppose it covers $x$ km using $25$ litres of petrol. The distance covered is directly proportional to the petrol used. Hence $x=25k$, where $k$ is the constant of proportionally. However we know that $432=36k$. (Why?) Hence
$k=\frac{432}{36}=12$.
Therefore
$x=25k=25\times 12=300km$.