Answer: $15385 should be deposited.
Explanation:
The principal was compounded monthly. This means that it was compounded 12 times in a year. So
n = 12
The rate at which the principal was compounded is 7.8%. So
r = 7.8/100 = 0.078
It was compounded for 4 years. Therefore,
t = 4
The formula for compound interest is
A = P(1+r/n)^nt
A = total amount in the account at the end of t years. The total amount is given as $21000. Therefore
21000 = P (1+0.078/12)^12×4
21000 = P (1+0.078/12)^48
21000 = P (1+0.0065)^48
21000 = P (1.0065)^48
P = 21000/1.365
P = $15385