84 Speed Math for Kids
Now, we have to place the decimal. How many digits are there after
the decimal in the problem? One, the 0 we provided. So we count
one digit back in the answer.
632.0 Answer
We would normally write the answer as 632.
Let’s check this answer using estimation. Eight is close to 10, so we
can round upwards.
10 × 79 = 790
Th e answer should be less than, but close to, 790. It certainly won’t
be around 7,900 or 79. Our answer of 632 fi ts, so we can assume it
is correct.
We can double-check by casting out nines.
8 × 79 = 632
8 7 2
Eight times 7 equals 56, which reduces to 11, then 2. Our answer
is correct.
Let’s try another.
98 × 968 =
We write 98 as 98.0 and treat it as 980 during the calculation. Our
problem now becomes 980 × 968.
980 × 968 =
–20
–32
Our next step is:
968 – 20 = 948
Multiply by the reference number:
948 × 1,000 = 948,000
1,000
brent
(Brent)
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