A number reduced by 25% becomes 225. What percent should it be increase so that it becomes 390 ?

Let the number be x
Then, x - 25% of x = 225
⇒ x - $$\frac{25}{100}$$x = 225
⇒ $$\frac{75x}{100}$$ = 225
⇒ x = $$\frac{225 × 100}{75}$$
⇒ x = 300
Required increase :
= (390 - 300)
= 90
∴ Increase % :
$$\eqalign{ & = \left( {\frac{{90}}{{300}} \times 100} \right)\% \cr & = 30\% \cr} $$