What sum would yield an interest of 36$ in 3 years at 3%p. a.

Amount of Interest = $36

Rate of interest = 3% per annum

Total time period for which the money was invested = 3 years

Let us assume the Principal amount = x

Then

Interest = Principal * Rate of Interest * time

36 = x * 3% * 3

36 = 9x/100

36 * 100 = 9x

3600 = 9x

x = 3600/9

= 400

So the initial sum of the money that was invested was $400.