mental math videos

https://www.youtube.com/watch?v=zKJf313iKNo - mental math - - So, youre 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?