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