When you get your check, (say it's $51.75), ask your kid to approximate 10%: it's right around $5.00. Then, tell your child to figure out a 20% tip! If he’s is stumped, tell him that 20% is twice as much as 10%, so double the $5.00 to about $10.00. Teaching your child to guesstimate gives him necessary math skills for the real world.