mental math - So, you’re at a restaurant, and the bill arrives.

Question; can you figure out the 15% tip in your head?

How about calculating the cost of 2 movie tickets, so you know you have enough cash for snacks left over?

Can you figure that out without a calculator?

How about this; If a cashier makes a $12.00 error on your grocery bill, can you realize it right away because you already ran the numbers in your head?