for compounded interest rate:
P=R(1+ni)n∗t , where P - current amount, R - initial amount, i - annual interest rate, n - number of payments in one year, t - number of years
a)
P=150000(1+20.025)2∗4≈165673
b)
P=150000(1+20.025)2∗8≈182983
c) Percent gained for 4 years: 150000165673−1≈0.1045=10.45 %
Percent gained for 8 years: 150000182983−1≈0.2199=21.99 %
So, additional interest earned due to the longer time is 21.99% - 10.45% = 11.54%
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