Doubling 15% of Judith's $70 bill to 14 dollars likely covers her 18% tip.
Here are two ways to calculate Judith's tip:
Method 1: Mental math:
Estimate the tip percentage: 18% is roughly between 15% and 20%.
Calculate a 15% tip: 70 dollars * 15% = 10.50 dollars.
Increase the tip by a third: Since 18% is slightly more than 15%, add 1/3 of the 15% tip: 10.50 dollars * (1/3) = 3.50 dollars.
Round up: Traditionally, tips are rounded up. Adding 3.50 dollars to the 15% tip gives a final tip of 14 dollars.
Method 2: Exact calculation:
Multiply the bill amount by the tip percentage: 70 dollars * 18% = 12.60 dollars.
Round the result: Tips are usually rounded to the nearest dollar. Rounding 12.60 dollars up gives a final tip of 13 dollars.
Therefore, Judith's tip amount could be either 13 dollars or 14 dollars, depending on your preferred rounding method.