For an approximation, we can start by rounding the number. In this case, you're probably expected to round to the nearest 1. 90.32 rounds down because 3 is less than 5. This leaves us with 90.
A good way to approximate this is to figure out what 10% is. You can do this by sliding the decimal over one place value.
10% of 90 is 9
10% + 10% = 20%
So we can add our two 10%'s together to find 20%
9 + 9 = 18
$18 is the tip
(I checked our work with the actual numbers and it checks out as a good approximation)
