Multiply 2 digit numbers with same tenth digit and the unit digits add up to 10

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Look upon the following example

Solve 23 x 27

Here you can see that the tenth digit of the of both 23 and 27 is the same (which is 2). And the unit digit of 23 (3) and 27(7) adds up to 10 (3+7 = 10). For such type of number, there is an easy way to multiply them.

1. Since the tenth digit of 23 and 27 is 2. We should multiply 2 with its successor (You will get the successor if you add the number with 1, which is 2+1 = 3) .   2 x 3 = 6
   6
2. The unit digits are 3 and 7. Multiply them and write it after the solution got on Step 1.  3 x 7 = 21
   621


23 x 27 = 621

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What if the Solution you got on Step 2 is a one-digit number. Let's take the following example!

Solve 31 x 39 

1. Since the tenth digit of 31 and 39 is 3. We should multiply 3 with its successor (You will get the successor if you add the number with 1) which is 3+1 = 4. 3 x 4 = 12
   12
2. The unit digits are 1 and 9. Multiply them and write it after the solution got on Step 1. 1 x 9 = 9. But it is just one digit. 129 is not the answer. You should pad a '0' in front of the answer. So, you should write 09 instead of 9.
   1209


31 x 39 =1209

Solve in your mind

13 x 17

25 x 25

14 x 16

94 x 96

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