MATH TRICK 5 says the short cut of multiplication between 11 and 20 but one of the numbers is a decimal.
Example 1
11.5 x 12
(The short cut of 11 x 12 has been studied in Maths Trick 1)
Here the base number is 10
Step 1. 11.5 + 2 = 13.5 (OR 12 + 1.5 = 13.5 1.5 and 2 are unit digits)
Step 2 13.5 x 10 = 135 ( 10 is the base number)
Step 3 1.5 x 2 = 3 (This is also explained in Maths Trick 1)
Step 4 135 + 3 = 138
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Yes . That is the answer.
Example 2.
12.5 x 13
12.5 + 3 = 15.5
15.5 x 10 = 155
2.5 x 3 = 7.5
Add up : 155 + 7.5 = 162.5
Yes . That is the answer.
Example 3.
15.4 + 1 = 16.4 x 10 = 164 +1 x 5.4 = 169.4
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Example 4
Calculation should be in the head.
17.2 + 2 =19.2 192 + 2 x 7.2 = 206.4
Exercise
1. 16.5 x 11
2. 19. 5 x 13
3. 18 x 13.5
4. 14 x 14.4
5. 15 x12.5
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