Hong Kong

Stage 1 - Stage 3

Lesson

We have looked at a lot of ways of calculating with whole numbers using mental computation and by hand methods. Now we are going to look at how we can do these kinds of calculations using a calculator.

I can hear you all saying, "Woohoo! I'll never have to do a mental calculation again!" However, this can be a risky assumption.

For example, try the following calculation on your calculator. Type it in exactly from left to right.

$6+3\times7$6+3×7

Some calculators will display $63$63, and others will display the correct answer of $27$27. This is because some calculators can correctly identify that, following the order of operations, the multiplication needs to be completed before the addition of $6$6.

So really, even though a calculator is a really helpful mathematical tool, it's still important to understand the maths behind what you're doing so you can make sure you the answer the calculator gives you is reasonable. When we're working with whole numbers, this may include the 4 main operations (addition, subtraction, multiplication and division), as well as the order of operations.

When you are faced with a mathematical question, you need to be able to look at it and decide whether it is something that you can do in your head (mental computation), by hand or if you need a calculator. This decision will be different for everyone, so you'll just have to try some questions out and see.

Remember!

It's not always faster to use a calculator!

e.g. You can probably tell me the answer to $1+1$1+1 faster than you can type it in a calculator.

Just another heads up- not all calculators and online devices work the same way, so it's important that you use the calculator you are allowed to use in your exams or tests often. I have seen many students struggle in a test because they borrowed a friends calculator at the last minute and they don't know how to use it!

Most high school students will have a scientific calculator that looks something like one of these:

Now let's say I wanted to know the answer to $351\times996$351×996.

Firstly, I may mentally estimate what my answer will be by rounding the numbers:

$350\times1000=350000$350×1000=350000

I know my answer should be around $350000$350000.

I would then enter the sum into the calculator by pressing each button individually in the exact order I need like so:

The display screen on my calculator showed me that the answer to this calculation is $349596$349596 (which is close to my estimate). So now we know the exact solution to this question.

Use a calculator to evaluate $214+443$214+443.

Using a calculator, evaluate $47-\left(14+\left(14\div7\right)\right)$47−(14+(14÷7))

Using a calculator evaluate $2^3\times3+81$23×3+81