The largest digit used in Hindu-Arabic number system is

What is regrouping in Math?

Work on the addition or subtraction problem, using regrouping as necessary. Convert the answer to Hindu-Arabic form.

X is a 2-digit number. When X is divided by 5, it has a remainder of 2. Another 2-digit number, Y, when divided by 20, has a remainder of 3. What is the largest possible difference between the two numbers? 

The average of three 2-digit number is 45. One of the numbers is 70. What is the largest difference between the other two numbers?