secrets-of-mental-math-malestrom

34
Secrets of Mental Math
problems like these in your head, and those that require you to
carry numbers will be almost as easy as those that don’t.
Rounding Up
You saw in the last chapter how useful rounding up can be
when it comes to subtraction. The same goes for multiplication,
especially when you are multiplying numbers that end in eight
or nine.
Let’s take the problem of 69 6, illustrated below. On the
left we have calculated it the usual way, by adding 360 54. On
the right, however, we have rounded 69 up to 70, and subtracted
420 6, which you might find easier to do.
69 (60 9) or 69 (70 1)
6
6
60 6 360
70 6 420
9 6 54
1 6 6
414
414
The following example also shows how much easier rounding
up can be:
78 (70 8) or 78 (80 2)
9
9
70 9 630
80 9 720
8 9 72
2 9 18
702
702
The subtraction method works especially well for numbers
that are just one or two digits away from a multiple of 10. It does
not work so well when you need to round up more than two dig-