The monthly salary of Mr. Sachdev gets increased by 5%, thereby his salary becomes 15,120 per annum. His earlier monthly salary (before the increase) was

Let annual salary of Sachdev before increase be Rs. x.

According to the question,

$x \times \frac{105}{100}=15120$

$\Rightarrow x=\frac{15120 \times 100}{105}$

$=$ Rs. 14400

$\therefore$ Required monthly salary

$=\operatorname{Rs} .\left(\frac{14400}{12}\right)=\text { Rs. } 1200$