📖Definition
Any two numbers can be multiplied together. This module deals with whole numbers and decimals.
Method
You should be familiar with the various techniques (grid method, column method).
💡Multiplying using decimals
Remove any decimal points but count the total number of figures after the points in the two numbers.
Do the multiplication and replace the decimal point so that the same number of figures as the total you counted are after the point.
💡Multiplying by 10, 100 or 1000
It can help to be able to quickly multiply by 10, 100 and 1000.
Multiplying a number by 10, 100 or 1000 is very straightforward.
If there's no decimal point in the original number, just add the same number of zeros after the number as there are 0s after the 1.
If there is a decimal point, then move the point the same number of places to the right as there are 0s after the 1.
For example, 6.9 x 100
There are two 0s in 100 so we move the decimal point two places to the right.
Moving it one place makes 69. Write this as 69.0 and move the point another place to the right for the second zero.
This gives 690.
💡Tips/hints
Often, estimating the answer of a multiplication can eliminate some wrong answers. For example, if you're asked to work out 8.9 x 7.1, round the two numbers
8.9 rounds to 9, 7.1 rounds to 7. The sum becomes 9 x 7 = 63.
So, any answers that aren't very close to 63 can be eliminated.
Often, when dealing with prices, we have to multiply a value that ends in .99 for example, if one book costs £2.99, how much do 7 books cost?
The sum is 7 x £2.99.
First calculate 7 x £3 = £21.
£3 = £2.99 + £0.01
So, we added £0.01 to each book which means we added £0.07 to the 7 books.
So, 7 x £2.99 = £21.00 - £0.07 = £20.93
✏️Example
Frank buys 22 pencils which cost 38p each. How much does he spend in total?
✅Solution
You need to do the sum 22 x 38. This should come to 836. Convert to £ for the answer.
You could estimate the answer by rounding 22 to 20 and 38 to 40.
The sum becomes 20 x 40 = 800. So you can then eliminate any of the answers that aren't close to 800.
Also, you can use this to check that your answer is in the right range.