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50 Speed Math for Kids
We now multiply the numbers in the circles:
2 × 14 = 28
It is actually –2 times 14 so our answer is –28.
640 – 28 = 612
To subtract 28, we subtract 30 and add 2.
640 – 30 = 610
610 + 2 = 612
Our problem now looks like this:
+14
20 18 × 34 = 32
–2
640 – 28 = 612 Answer
2
Let’s check the answer by casting out the nines:
18 × 34 = 612
9 7 9
0 0
Zero times 7 is 0, so the answer is correct.
USING 50 AS A REFERENCE NUMBER
Th at takes care of the numbers up to around 30 times 30. What if
the numbers are higher? Th en we can use 50 as a reference number.
It is easy to multiply by 50 because 50 is half of 100. You could
say 50 is 100 divided by 2. So, to multiply by 50, we multiply the
number by 100 and then divide the answer by 2.