It is easy to add 948,000 and 640.
948,000 + 640 = 948,640
We now have to adjust our answer because we were multiplying by 98, not 980.
Placing one digit after the decimal (or dividing our answer by 10), we get 94,864.0, which we simply
write as 94,864.
Our full working would look like this:
Let’s cast out nines to check the answer.Five times 8 is 40, which reduces to 4. Our answer, 94,864, also reduces to 4, so our answer seems to
be correct.
We have checked our answer, but casting out nines does not tell us if the decimal is in the correct
position, so let’s double-check by estimating the answer.
Ninety-eight is almost 100, so we can see if we have placed the decimal point correctly by
multiplying 968 by 100 to get 96,800. This has the same number of digits as our answer so we can
assume the answer is correct.
Test yourself
Try these problems for yourself:
a) 9 × 82 =
b) 9 × 79 =
c) 9 × 77 =
d) 8 × 75 =
e) 7 × 89 =
The answers are:
a) 738
b) 711
c) 693
d) 600
e) 623
That was easy, wasn’t it? If you use your imagination you can use these strategies to solve any
multiplication problem. Try some yourself.