j) 224
If any of your answers were wrong, read through this section again, find your mistake, then try again.
How would you solve 13 × 21? Let’s try it:We still use a reference number of 10. Both numbers are above 10 so we put the circles above.
Thirteen is 3 above 10, 21 is 11 above, so we write 3 and 11 in the circles.
Twenty-one plus 3 is 24, times 10 is 240. Three times 11 is 33, added to 240 makes 273. This is how
the completed problem looks:
Multiplying Numbers Above 100
We can use our speed maths method to multiply numbers above 100 as well. Let’s try 113 times 102.
We use 100 as our reference number.
Add crossways:
113 + 2 = 115
Multiply by the reference number:
115 × 100 = 11,500
Now multiply the numbers in the circles:
2 × 13 = 26
This is how the completed problem looks:Solving Problems in Your Head
When you use these strategies, what you say inside your head is very important, and can help you solve
problems more quickly and easily.
Let’s try multiplying 16 by 16.
This is how I would solve this problem in my head:
16 plus 6 (from the second 16) equals 22, times 10 equals 220
6 times 6 is 36
220 plus 30 is 250, plus 6 is 256
Try it. See how you go.