To calculate tip, you need to multiply the total bill by the percent tip you are going to leave. In this case the bill is $136.40, and the customer is going to leave a 15% tip. So, you would want to move the decimal place over to the left by 1, which you should get 13.64 as your number. Then divide it in half, which should give you 6.82. Now add 13.64 and 6.82 with the original bill price. If you don't add the 13.64 you would've forgotten the 10% part of the tip. At the end you should get $156.86 as your final bill.
