Question

A plane increases its cruising speed from 400mph to 550mph. Work out the percentage speed increase.

Answer 1

Well, it went from 400 to 550, so the difference is 150.

if we take 400 to be the 100%, what is 150 off of it in percentage?

$\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 400&100\\ 150&p \end{array}\implies \cfrac{400}{150}=\cfrac{100}{p}\implies p=\cfrac{150\cdot 100}{400}$