Petra deposited $278.16 in a savings account that earns 2.1% simple interest. what is petra’s account balance after nine years?

The answer is $330.73.

Step 1. Calculate simple interest: I = ?
Step 2. To get account balance add simple interest to the principal amount:
I + P = ?

Step 1. Calculate simple interest.
I = P * r * t
P - principal
r - rate
t - time period

We have:
P = $278.16
r = 2.1% = 2.1/100 = 0.021
t = 9

I = 278.16 * 0.021 * 9 = 52.57

Step 2. To get account balance add simple interest to the principal amount:
I + P = ?
52.57 + 278.16 = 330.73