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.