How to Do Number Sense (Mental Math)

Step-by-Step Guide

1. 1

   Step 1: Convert difficult to add numbers to easy to add numbers.

   Round the number (to be added) up to the next highest multiple of ten.
   
   Add to the other number.
   
   Subtract the amount rounded up.
   
   Example 88 + 56 = ? ; Round 88 up to
   90.
   
   Add 90 to 56 = 146 Subtract the two added to 88 (to round up to 90). 146
   - 2 = 144 ; the answer! This process is simple reframing of the problem as 56 + (90
   -2 ).
   
   Examples of other usages of this technique: 99 = (100
   - 1); 68 = (70
   - 2) You can use a similar reframing technique for subtraction, too.
2. 2

   Step 2: Convert addition to multiplication.

   Multiplication is addition of multiple occurrences of the same number.
   
   Note how many times a number to be added is repeated.
   
   For example: 7 + 25 + 7 +7 +7 = becomes 25 + (4 × 7) = 25 + 28 = 53 , Additive opposites can be +7
   -
   7.
   
   Additive opposites can also be 5
   - 2 + 4
   -
   7.
   
   Look for numbers that add or subtract for a total of
   0.
   
   Using the example above: (NOTE:
   The picture above is wrong.
   
   It shows 5 + 9 = 9 <-->
   -2
   -7 = 9 when it should be 5 + 4 = 9 <-->
   -2
   -7 =
   -9) 5 + 4 = 9 is the additive opposite of
   -2
   -7 =
   -9 Since they are additive opposites, no actual addition of all four numbers is needed; the answer is 0 (zero) by cancelling.
   
   Try this: 4 + 5
   - 7 + 8
   - 3 + 6
   - 9 + 2 = becomes:(4 + 5)
   - 9 + (
   -7
   - 3) + (8 + 2) + 6 = By grouping and remember, don't add them; just remove additive opposites from the problem. 0 + 0 + 6 = 6
3. 3

   Step 3: Cancel Additive Opposites.

Detailed Guide

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