explain how you could use mental math to figure out how much a 20% tip would be for a total restaurant bill of $22.50

Question

explain how you could use mental math to figure out how much a 20% tip would be for a total restaurant bill of $22.50

Answer 1

20% of $22.50 is a little more than 1/5 of $20 which is $4. So $4.50 should be close to 20%

1/5 of $20 is $4. 1/5 of the extra $2.50 is 2.5/5 = .50 = 50 cents. $4 + 50 cents = $4.50

Most people round off the tip, rather than calculate to the nearest cent. but in this case it comes out exactly the same.

20% is 0.20

multiply by 2 to get $45, then move the decimal to the left one digit to get $4.50

multiply by 2 then divide by 10 is the same as multiplying by 0.20