To multiply any number by 10, 100 or 1000, just add to the first multiplier the number of zeros from the 10/100/1000.
For example, to multiply 12 by 10, you need to add zero from the multiplier 10 to the multiplier 12. The answer is 120
More examples:
12 x 100 = 1200 (two zeros were added to 12 because there are two zeros in the number 100)
12 x 1000 = 12000 (add three zeros to 12, because there are three zeros in the number 1000)
15 x 100 = 1500 (two zeros were added to 15, because there are two zeros in number 100)
320 x 100 = 32000 (two zeros were added to 320, because there are two zeros in 100)
If it is not the multiplier that ends with zero, but the other one, then you should add zero after the second multiplier in order to get the answer.
For example, to multiply 10 by 12, you should write in the answer the multiplier 12 and add one zero at the end:
10 x 12 = 120
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