The account has a balance of $64.98 at the start. That means that there is $64.98 in the account.
A deposit of $73.87 is made after which the amount in the account is $64.98 + $73.87 = $138.85.
Next come two withdrawals (this is money that is taken out of the account so we need to subtract the amounts given). $138.85 - $51.00 = $87.85 and $87.85 - $23.50 = $64.35.
Interest is added to the account. So we end by adding $1.15 to the amount we just arrived at. That is, $64.35 + $1.15 = $65.50
