A sum of Rs. 6,240 is paid off in 30 installments in such a way that each installment is Rs. 10 more than the previous installment. The first installment would be of

The correct answer is (c).

Let $x$ be the first installment. The total amount of the installments is $6240$, so the sum of the first 29 installments is $6240-x$. The 29th installment is $x+29$, and the 28th installment is $x+28$. The 29th installment is $10$ more than the 28th installment, so $x+29=x+28+10$. Simplifying, we get $x=37$.

Option (a) is incorrect because $36$ is not a multiple of $10$. Option (b) is incorrect because $63$ is not a multiple of $10$. Option (d) is incorrect because $73$ is not a multiple of $10$.