Mathematics, Operations (+, -, x, /, etc.), Problem Solving

Grade 5- 8

Objective

Students learn methods for easy mental multiplication.

Directions

Tell students that it is possible to multiply large numbers in their heads using some simple techniques. Explain each technique to them and write the examples on the board as you go along.
Multiplying by 2 just means doubling the number.
Example: 340 x 2 = 680
Multiplying by 5 is done by multiplying by 10 and cutting the answer in half.
Example: 660 x 5 = ?
660 x 10 = 6,600
6,600 ÷ 2 = 3, 300
660 x 5 = 3, 300
Multiplying by 9 is done by multiplying by 10 and subtracting the other factor.
Example: 63 x 9 = ?
63 x 10 = 630
630 - 63 = 567
63 x 9 = 567
Multiplying by 50 is done by multiplying by 100 and dividing by 2.
Example: 34 x 50 = ?
34 x 100 = 3, 400
3,400 ÷ 2 = 1,700
34 x 50 = 1,700
Multiplying by 25 is done by multiplying by 100 and dividing by 4.
Example: 28 x 25 = ?
28 x 100 = 2800
2,800 ÷ 4 = 700
28 x 25 = 700

Resources

• Easy Mental Multiplication activity sheet (one per student)
• pencils