Hong Kong
Stage 2

# Multiply a single digit number by a four digit number using algorithm

Lesson

When we multiply a single digit number, such as 4, by a four digit number, such as 1423, we can break this into four separate multiplications.  Then, all we need to do is add those four answers together.   There are also some other methods that can help, so you can choose the method that works for you.

#### Worked Examples

##### Question 1

Calculate these multiplications.

1. $3\times3$3×3

2. $3\times30$3×30

3. $3\times300$3×300

4. $3\times3000$3×3000

##### Question 2

Calculate $2\times3482$2×3482 by doing the following.

1. First find $3482+3482$3482+3482 by filling in the addition table.

You may need to carry if the digits in a column add to more than $9$9

 $\editable{}$ $3$3 $4$4 $8$8 $2$2 $+$+ $3$3 $4$4 $8$8 $2$2 $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$
2. Now find $3482\times2$3482×2 by filling in the multiplication table.

You may need to carry over into the next column if your digits in a column add to more than $9$9.

 $\editable{}$ $3$3 $4$4 $8$8 $2$2 $\times$× $2$2 $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$

##### Question 3

Calculate $3\times2518$3×2518 by doing the following.

1. First find $2518+2518+2518$2518+2518+2518 by filling in the addition table.

 $\editable{}$ $\editable{}$ $2$2 $5$5 $1$1 $8$8 $+$+ $2$2 $5$5 $1$1 $8$8 $+$+ $2$2 $5$5 $1$1 $8$8 $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$
2. Now find $2518\times3$2518×3 by filling in the multiplication table.

You may need to carry digits over into the next column.

 $\editable{}$ $\editable{}$ $2$2 $5$5 $1$1 $8$8 $\times$× $3$3 $\editable{}$ $\editable{}$ $\editable{}$ $\editable{}$