1276 – 165 =
3 916 – 817 =
4 96 – 58 =
5 117 – 99 =
6 2139 – 126 =
7 6483 – 5261 =
8 2912 – 1915 =
9 792 – 689 =
10 542 – 454 =
11 If a patient’s total fluid input over 24 hours was 3205 millilitres and their output was 2410 millilitres, what is their overall fluid balance?
12 A patient takes morphine sulphate solution 10 mg in 5 millilitres orally, six times each day. The ward stock bottle contains 250 millilitres. How much will be left in the bottle after seven days?
2.5 Multiplication
Multiplication is the same as repeatedly adding the same number together, for example, 5 × 12 is the same as 12 + 12 + 12 + 12 + 12, but the process is less time consuming. Multiplications are used to calculate fluid balance charts where the same volume has been given repeatedly and when calculating drug doses that are prescribed per kilogram of body weight.
Many people remember their ‘times tables’ from school. If you didn’t learn these or have forgotten them, you might find that a copy of the ‘multiplication grid’ in Appendix 5 acts as a valuable resource. If you don’t know your times tables, it’s worth practising them until you are confident that you can do simple multiplications in your head.
When multiplying two or more numbers together, the calculation can be performed in any order, so 25 × 4 gives the same answer as 4 × 25.
To check your multiplications
Divide your answer by one of the numbers you multiplied.
For example 6 × 4 = 24 therefore 24 ÷ 4 = 6.
| EXAMPLE 2.7 |
Method
The multiplication is calculated vertically from right to left, starting under the ‘ones’ column and involves three individual calculations, one for the ‘ones’ column, one for the ‘tens’ column and a final calculation for the ‘hundreds’ column.
Process
This gives the answer of 4 hundreds, 6 tens and 8 ones, or 468.
Checking
To check your answer: 468 ÷ 2 = 234.
Not all multiplications are as straightforward as this. There will be times when the calculation results in ten or more in a column, such as in Example 2.8.
| EXAMPLE 2.8 |
Method
As before, the multiplication is calculated vertically from right to left, starting under the ‘ones’ column and involves three individual calculations, one for the ‘ones’ column, one for the ‘tens’ column and a final calculation for the ‘hundreds’ column.
Process
The number ‘15’ is made up of one ‘ten’ and five ‘ones’. Because this column is only used to record the ‘ones’, the five ‘ones’ are recorded here and the ‘ten’ carried over to the ‘tens’ column. The usual way of doing this is to write a small ‘1’ below the 2 under the ‘tens’ column.
This gives the answer of 3 hundreds, 7 tens and 5 ones, or 375.
Checking
To check your answer: 375 ÷ 3 = 125.
This process works well when the multiplication involves one number that is less than ten. When both of the numbers to be multiplied together are greater than ten, an extra stage of calculation is necessary.
| EXAMPLE 2.9 |
A fluid balance chart shows that a patient’s oral intake in one 24-hour period was 11 glasses of water. Each glass measures 85 ml. What is the total oral intake?
This calculation involves multiplying 85 and 11.
Method
As before, the multiplication is calculated vertically from right to left, making sure that individual digits are kept in position within the columns. The terms ‘ones’, ‘tens’ or ‘hundreds’ column used within the explanation only refer to the top number in the calculation.
Process
The multiplication has two stages, firstly multiplying the top number by the 1 ‘one’ belonging to the 11 and secondly multiplying the top number by the 1 ‘ten’ belonging to the 11.
Stage one
Stage two
Stage two starts by placing a ‘zero’ under the ‘ones’ column to maintain the place of the other digits. This is done because the number being used in the multiplication from the bottom line is a ten, not a ‘one’.
The results of the two separate multiplications are then added together:
So the total oral intake of fluid over the 24-hour period is 935 ml.
Checking
To check your answer: 935 ÷ 11 = 85.
TOP TIP
Remember that:
an even number multiplied by an even number always makes an even number
an even number multiplied by an odd number always makes an even number
an odd number multiplied by an odd number always makes an odd number.
Self-assessment test 2.4: multiplication
