How do you multiply?
Worked examples: Multiplication
Multiplication is the process of adding a number to itself several times.
Example
You order 32 packets of 10mm bolts 20mm long and each packet is $37. What will be the total cost for 32 packets?
Set out the problem, making sure to keep UNITS, TENS, HUNDREDS etc aligned.
3 7
x 3 2
First work with the UNITS column of the number you are multiplying by.
Multiply 37 by 2.
37 x 2 = 74
Write your result under the line, being careful to keep the columns aligned.
3 7
x 3 2
7 4
Now you need to multiply by the number in the TENS column.
Place a zero in the UNITS column under the line and your multiplication becomes.
37 x 3 =?
3 7
x 3 2
7 4
0
Multiply 7 by 3.
7 x 3 = 21
Write the 1 under the line and carry the 2 up to the TENS column.
327
x 3 2
7 4
1 0
Multiply 3 by 3 and add the carried 2.
3 x 3 = 9 and then 9 +2 = 11.
Write the result under the line.
327
x 3 2
7 4
1 1 1 0
Now add 74 and 1110.
3 7
x 3 2
7 4
1110
1184
Our completed answer to the total cost of 32 packets is $1,184.
Multiplication problems
1. You need to buy 5 metres steel pipe at $17 per metre. What will this cost?
2. What is the minimum length of bench needed to lay out end to end 13 pieces of flat bar each 178mm long?
3. You have to cut 23 pieces of flat bar, making each 17mm long. What will be the total length of flat bar you need?
4. You have fabricated 5 small containers and each has an external height of 270mm. If you stack the 5 boxes on top of each other, how high will the top surface of the stack be from the ground?
5. You are going to store some lengths of pipe with an outside diameter of 65mm in a stacking rack. What width will you need if you want to fit 17 lengths in a single layer?
Check your responses with the answers below.
Answers
(1) $85 (2) 2314mm (3) 391mm (4) 1350mm (5) 1105mm
Activity sheet: How do you multiply Page 1