The bill without tip is $48.00
To calculate the total bill with tip, considering that you want to leave a 15% tip, you have to calculate how much is the 15% of $48.00. To do so, multiply 48 by 0.15:

The amount that corresponds to a 15% tip is $7.20
To calculate the total bill, tou have to add the bill plus the tip

The total bill will be $55.20