3. Multiplication and division

Lesson

We can multiply numbers by larger numbers, once we've mastered multiplying by single digit numbers .

Find 6178 \times 4.

Worked Solution

Idea summary

We can use several different methods to solve multiplication problems, but we don't always know if we will need to do any regrouping or trading. If we use an algorithm, regrouping can be done as we solve our problem.

Let's see how to multiply a 2 digit number by another 2 digit number.

Let's use the area model to find 28\times 15.

a

Find the area of each rectangle.

Worked Solution

b

Find the sums of each column.

Worked Solution

c

What is the total area of the rectangles?

Worked Solution

d

Find 28\times15.

Worked Solution

Idea summary

Using a standard algorithm means we can multiply larger numbers the same way, just with more steps. Once we know how to solve problems this way, there's no limit to how big the numbers could be.

What if we have larger numbers? Let's see how we can multiply larger numbers by 1 and 2 digit numbers.

Find the product of 243 \times 15.

Worked Solution

Idea summary

When multiplying by a two digit number using a vertical algorithm:

Multiply by the units digit first.

Put a 0 in the units column and then multiply by the tens digit.

Add the two products together to get the final answer.