Success on the Numeracy Test: Multiplying and Dividing by Decimals - dummies

Success on the Numeracy Test: Multiplying and Dividing by Decimals

By Colin Beveridge, Andrew Green

Part of Teacher’s Skills Tests For Dummies Cheat Sheet (UK Edition)

In the mental arithmetic test, you’re often asked to multiply or divide a number by a decimal – something like 10.3 x 0.01 or 3.34 ÷ 0.2. Once you know the rules, they’re easy!

To multiply by a decimal number:

  1. Count how many digits are after the decimal point (the dot) in both numbers. In the example 10.3 x 0.01, the first number has one digit after the decimal point, and the second has two.

  2. Add these numbers up (here, you get three) and write the answer down somewhere.

  3. Ignoring the dot, work out the sum and put a dot at the end: 103 x 1 = 103.

  4. Move the dot back the number of digits you wrote down in Step 2 – there’s your answer! Here, you’d move the dot back three digits and end up with 0.103.

Sometimes, you’ll need to move back more places than you have digits – in that case, put as many zeroes as you need in front of the answer in Step 3.

Dividing by a decimal number is easier!

  1. Count how many digits are after the decimal point (the dot) in both numbers. In the example 3.34 ÷ 0.2, the first number has two digits after the decimal point, and the second has one.

  2. Multiply both numbers by 10 until the bottom is a whole number. Here, you need to do that once to get 33.4 ÷ 2.

  3. Do the sum! Your answer is 16.7.