Teaching mathematics

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

# 2.3 Mental multiplication and division strategies

Doubling and halving are easy for most people, so make use of these tactics:

• doubling and doubling again is the same as multiplying by 4

• doubling and doubling and doubling again is the same as multiplying by 8.

Use the inverse of doubling (i.e. the opposite of doubling) which is halving:

• halving is the same as dividing by 2

• halving and halving again is the same as dividing by 4

• halving and halving and halving again is the same as dividing by 8.

Once we begin talking about halving being the same as dividing by 2 this connects to fractions, decimals and percentages. There will be more of this in Week 2 but for now:

• half of 100% is 50% → to find 50% of an amount just half it

• half of 50% is 25% → to find 25% of an amount half and half it again.

## Activity 5 Finding halves and quarters

Timing: Allow 5 minutes

Try the following questions.

### Discussion

Listed below are some useful mental multiplication strategies:

Multiply by 5 by multiplying by 10 then halving

Multiply by 20 by multiplying by 10 then doubling (or vice versa)

Multiply by 50 by multiplying by 100 and halving (or vice versa)

Multiply by 11 or 9 by multiplying by 10 and adding or subtracting

Multiply by 21 or 19 by multiplying by 20 and adding or subtracting

TM_1

### Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371