What sum of money will amount to Tk. 1500 in 5 years and to Tk. 1620 in 7 years at simple interest?

Question: What sum of money will amount to Tk. 1500 in 5 years and to Tk. 1620 in 7 years at simple interest?

Solution:
in two years the interest becomes = 1620 - 1500 = 120Tk.
in 5 years, interest = (120/2)/5 =300 Tk.

so , the principle amount is = 1500 - 300 = 1200 Tk.