There is $25\%$ profit if an article is sold at Rs. $150$. At what per cent should the selling price be increased so that there will be $30\%$ profit?

Let the cost price of the article be $\text{CP}$.
Given that the article is sold at Rs. $150$ with a profit of $25\%$.

$${\text{SP}}_1=\text{CP}\times \left(1+\frac{25}{100}\right)=\text{CP}\times 1.25$$

$$150=\text{CP}\times 1.25⟹\text{CP}=\frac{150}{1.25}=\text{Rs. }120$$

To get a profit of $30\%$, the new selling price (${\text{SP}}_2$) should be:

$${\text{SP}}_2=\text{CP}\times \left(1+\frac{30}{100}\right)=120\times 1.30=\text{Rs. }156$$

The increase in selling price is:

$$\Delta \text{SP}={\text{SP}}_2-{\text{SP}}_1=156-150=\text{Rs. }6$$

The percentage increase in the selling price is:

$$\text{Percentage Increase}=\left(\frac{\Delta \text{SP}}{{\text{SP}}_1}\right)\times 100\%=\left(\frac{6}{150}\right)\times 100\%=4\%$$