Number Tricks. How to work out the product of two numbers that differ by 2 (mental math tricks).

Here I will show you a number trick that can be used to multiply two numbers that have a difference of 2. For example, 17 × 19, 22 ×20, 41 × 43... and so on. The only downside is that you need to know your square number well. Here are the first twenty square numbers to help you out:

1,4,9,16,25,36,49,64,81,100,121,144,169,196,225, 256, 289, 324, 361, 400...

All you need to do then is follow these two steps:

Step 1 Work out the number in the middle of the two numbers and square this number.

Step 2 Subtract 1 from your answer in step 2.

Let’s try these two steps out on some examples:

Example 1

Work out 11 × 13

Step 1 Work out the number in the middle of the two numbers and square this number.

The number in the middle of 11 and 13 is 12 and 12 squared is 144.

Step 2 Subtract 1 from your answer in step 2.

144 – 1 = 143

Example 2

Work out 16 × 14

Step 1 Work out the number in the middle of the two numbers and square this number.

The number in the middle of 14 and 16 is 15 and 15 squared is 225.

Step 2 Subtract 1 from your answer in step 2.

225 – 1 = 224

Example 3

Work out 17 × 19

Step 1 Work out the number in the middle of the two numbers and square this number.

The number in the middle of 17 and 19 is 18 and 18 squared is 324.

Step 2 Subtract 1 from your answer in step 2.

324 – 1 = 323

Example 4

Work out 79 × 81

Step 1 Work out the number in the middle of the two numbers and square this number.

The number in the middle of 79 and 81 is 80 and 80 squared is 6400.

Step 2 Subtract 1 from your answer in step 2.

6400 – 1 = 6399

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