Multiplication Roundabouts

This page shows another way to mentally multiply a two-digit number by a one-digit number. In this strategy, students round the two-digit number up or down to the nearest 10 to make the number easier to work with and multiply it by the one-digit number. Then they add  or subtract the amount they rounded, multiplied by the one-digit number, to find the correct answer.
Encourage the students to solve the problems in their head and not write the working down. If they are finding this difficult, you could give them smaller numbers to multiply to begin with, or they could work in pairs. Ensure that they always round the two-digit number, not just the second number in the problem. For example, in questions g and h, the number to be rounded is the first number in the problem rather than the second, unlike the other problems.

Answers to Activity

a. 2 x 80 = 160 2 x 2 = 4 160 – 4 = 156
b. 5 x 30 = 150 5 x 2 = 10 150 + 10 = 160
c. 4 x 40 = 160 4 x 1 = 4 160 – 4 = 156
d. 9 x 40 = 360 9 x 1 = 9 360 + 9 = 369
e. 3 x 50 = 150 3 x 4 = 12 150 + 12 = 162
f. 8 x 60 = 480 8 x 1 = 8 480 – 8 = 472
g. 6 x 90 = 540 6 x 2 = 12 540 – 12 = 528
h. 7 x 30 = 210 7 x 1 = 7 210 – 7 = 203