Idiot\'s Guides Basic Math and Pre-Algebra

74 Part 1: The World of Numbers

For example, here’s how you add 34.82 and 9.7.

Align the decimal points and add a zero to 9.7.

Add as you normally would. The hundredths place adds to 2 hundredths, so you can just put that

down, but the tenths place adds to 15 tenths, so you need to carry 1 to the ones place. You’ll carry

again from ones to tens. You work right over the decimal points as though they weren’t there.

Bring the decimal point straight down when you’re all done.

Subtraction works the same way. Align the decimal points, and add zeros to make the decimals

the same length. Those zeros are especially important in subtraction, because you can’t subtract

from a digit that isn’t there. Subtract normally, regrouping if you need to, and bring the decimal

point straight down. Let’s try a subtraction example.

Subtract 184.93 – 99.781

Align the decimal points and add a trailing zero to 184.93. Subtract, starting at the right.

Regroup or borrow as you need to (and you need to a lot for this one). Bring the decimal point

straight down.

You can check your answer to a subtraction problem by adding. In the example above, check to

see if 85.149 + 99.781 gives you 184.930. If it does, your subtraction is correct.

WORLDLY WISDOM

Taking a minute to estimate you answer before you do a calculation is always a good

idea, and with decimals, it will help you be certain you got the decimal point in the

right place. It doesn’t need to be anything fancy. If you can say that you’re subtracting

about 185 – 100, you’ll know you should be getting an answer around 85. If you get an

answer more like 8 or 850, you’ve misplaced the decimal point.

34.82

9.70

11

44.52

18 4 9 3 0

9 9 7 8 1

(^718211)
85 14 9
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