An easier way to do that is: Consider 10% of 80 is 8, so 4 times that would be 32. This is similar to the way I calculate a tip when eating out. For example, if the total bill is $15.75, and you want to leave 20% tip, just figure 10% of $15.75 is $1.57, double that to get 20% and you can leave (roughly) a $3 tip. Hope this helps.
