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80 Speed Math for Kids
tells us our decimal is in the right place. Th is is a good double-check.
You should always make this check when you are multiplying or
dividing using decimals. Th e check is simply: does the answer make
sense?
Let’s try another.
9.6 × 97 =
We write the problem down as it is, but work it out as if the numbers
are 96 and 97.
100 9.6 × 97 =
–4
–3
96 – 3 = 93
93 × 100 (reference number) = 9,300
4 × 3 = 12
9,300 + 12 = 9,312
Where do we put the decimal? How many digits follow the decimal
in the problem? One. Th at’s how many digits should follow the
decimal in the answer.
931.2 Answer
To place the decimal, we count the total number of digits following the
decimals in both numbers we are multiplying. We will have the same
number of digits following the decimal in the answer.
We can double-check the answer by estimating 10 times 90; from
this we know the answer is going to be somewhere near 900, not
9,000 or 90.
If the problem had been 9.6 × 9.7, then the answer would have
been 93.12. Knowing this can enable us to take some shortcuts