The formula for simple interest is:
Where
i is the interest earned
P is the principal
r is the rate of interest, in decimal
t is the time, in years
First,
21 months is:
21/12 = 1.75 yr
So, t = 1.75
Account A:
i = 10.50
r = 0.04
t = 1.75
Thus, principal would be:
Account B:
i = 15.75
r = 0.03
t = 1.75
Thus, principal would be:
The correct answer is A.
Account B because principal for Account B is $300 and the principal for Account A is $150.