Tips the Mental Math Way

Finn reads this

A tip is extra money you give for good service, usually a percent of the bill. People work tips out in their heads, and this is how.

Twenty percent on $35

Finn reads this

The meal is $35 and you want to leave 20 percent. Ten percent of $35 is $3.50: move the decimal point one place left. Twenty percent is double that: $7. The total is $35 plus $7, which is $42.

ElleMath-speak

A 20 percent tip is 0.20 times the bill.

FinnPlain English

Find 10 percent by moving the decimal, then double it.

Fifteen percent

Finn reads this

For 15 percent, find 10 percent and add half of it. On a $24 bill, 10 percent is $2.40 and half of that is $1.20, so the tip is $3.60 and the total is $27.60. The multiplier way agrees: 24 times 0.15 is $3.60, and 24 times 1.15 is $27.60.

For 18 percent: 10 percent, plus 5 percent, plus 1 percent three times. Or just multiply by 0.18. The mental math is for the restaurant; the multiplier is for the test.

Let me try a few

NC standards: NC.7.RP.3.

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