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.