18 months ago, a sum of money was invested. Now the investment is worth RM10,900. If the investment is extended for another 20 months, it will become RM11,900. Find the original principal and the simple interest rate that was offered.
If the interest rate is r, and the amount invested is x, then
x(1 + r/12 * 18) = 10900
so another 20 months will yield
x(1 + r/12 * 38) = 11900
Solve those and you get
x = 10,000
r = .06 = 6%