If I put $300 in an account that earns 5.5%, how much will there be in the account at the end of 21 years?

The simple interest formula is the following:

A=P (1+rt)

A=amount accumulated (or how much there will be in the account at the end of 21 years)

P=Principal (how much money we start with. In this case is $300)

r=interest rate in decimals (0.055 in this case. We got this number by converting 5.5% into decimals. You do this by dividing 5.5 by 100)

t=time. 21 years in this case

Let's now go ahead and solve for A:

A=300 (1+0.055*21)

0.055*21=1.155

A=300 (1+1.155)

A=300 (2.155)

A=646.50

At the end of 21 years you will have $646.50 in your bank account.