Split your classroom up into teams. Each team will have a restaurant patron and a waitress (or waiter). Give each team a slip showing what they had for dinner and the sum of what they owe. Make these different slips for every team.

The goal is to calculate a 15% tip using mental math. Once the restaurant patron calculates the tip, the waitress or waiter has to indicate whether she or he is happy or mad.

For example, let’s say the dinner tab came up to $35.75.
The patron can round that number to $36.
With mental math, 10% of $36 is $3.60 and 5% more is $1.80.

$3.60 + $1.80 = $5.40 so the waitress should be happy with $5.40 or above. (the actual calculation is $5.36 so the mental math gets you close)