Final answer:
To approximate a 15% tip for a bill of $31.99, find 10% of the bill, halve it (5%), and add both amounts to get the approximate tip. The best answer is $4.80.
Step-by-step explanation:
To approximate a 15% tip for a bill of $31.99, you can use mental math. First, find 10% of the bill by moving the decimal point one place to the left, which is $3.19. Then, find half of 10% (5%) by dividing $3.19 by 2, which is approximately $1.60. Finally, add 10% and 5% to get the approximate 15% tip. $3.19 + $1.60 = $4.79.
Therefore, the best answer from the choices provided is $4.80 (b).
Learn more about Mental math for calculating tips