Your lunch bill is $9.87 plus tax. How much should you tip? Use 20%

The tip should be 2.70 i believe i also did mental math.

Heres a really simple way to calculate the percent of a number when the percent is a multiple of 10 (10%, 20%, 30% etc.)

take the number, in your case $9.87

now move the decimal 1 spot to the left

this will give you $0.987 or 98.7 cents

this number is 10% of the total value of the number

to get 20% of the number, simply double it

.987*2 = $1.974

when dealing with money and other things in which it is not possible to split up into fractions of the object (you cannot have a fraction of a penny) you round down to the nearest cent

in this case, round to $1.97