mental math
Mental Math Tricks - Addition, Subtraction, Multiplication & Division!
03 - Mental Math Secrets! - The Secret to Mental Addition - Math Tricks for Fast Calculations!
How to Calculate Faster than a Calculator - Mental Maths #2| Addition and Subtraction
How to Calculate Faster than a Calculator - Mental Maths #1
Mental Math Tricks - Addition and Subtraction in your head!
Fast Mental Multiplication Trick - multiply in your head using base 10