If annual rate of simple interest increases from 10% to 15%, a man's yearly income increases by Rs.4000.His principal in(Rs)

To find the principal amount, we need to understand that the increase in interest rate from 10% to 15% is the reason for the Rs.4000 increase in the man's yearly income. This means that the 5% increase in interest rate is equivalent to Rs.4000.

Step 1: Express the increase in interest rate as a decimal. 5% = 0.05

Step 2: Set up the equation to find the principal (P). The formula for simple interest is I = PRT, where I is interest, P is principal, R is rate, and T is time. In this case, the increase in interest (I) is Rs.4000, the rate (R) is the increase in rate (0.05), and the time (T) is 1 year. So, the equation is 4000 = P * 0.05 * 1.

Step 3: Solve the equation for P. P = 4000 / (0.05 * 1) = Rs.80000.

So, the principal amount is Rs.80000.