#### Explanation:

It is very easy to estimate 10% of any number - just divide by 10 by moving the decimal point one place to the left.

Once you know 10% of an amount, it is possible to find 5% by dividing the value that is 10% by 2. Adding these answers gives a quick estimate of 15%

For example: "Find 15% of 300"

10% of 300 = 30 and $30 \div 2 = 15$

15% of 300 = 30+15 =45

In the same way for this question ...

Treat $89.50$ as $90 Find 10%, then 5%. 10% +5% =$9+%4.50 = $13.50 15% of the bill is about $13.50