File: Interactive flash presentation
Today, we will not use paper and pencil to find the difference but instead we will try to compute it using our mind.
Think of how we subtract numbers. In what direction do we follow when we subtract number?
Subtract number mentally the ones, then the tens.
Here are other ways of subtracting mentally:
We can use compensation:
Subtract: 34 - 19
THINK 20 is 1 more than 19
34 - 20 = 14, so 1 more is 15
We can add the same digit to both numbers:
Subtract: 42 – 18
THINK: Add 2 to both numbers
(42 + 2) - (18 + 2)
44 - 20 = 24