M3.1.4 For the adult
Quick tests for dividing
How can you tell if a number divides exactly, without doing the division?
What this chapter teaches
The last digit decides whether a number divides by 2, 5 or 10. For 3 and 9, add up all the digits and look at the digit sum. These tests save a long division.
Three questions to ask
There is no one right wording. Listen for the idea.
- How can you tell that 3 840 divides by 10 without dividing?
- What is the digit sum of 5 817, and is 5 817 in the 3 times table?
- Why does the last digit not decide whether a number divides by 3?
After the chapter, the learner can
- Use the last digit to test for 2, 5 and 10
- Use the digit sum to test for 3 and 9
What they copy into their notebook
Ends in 0, 2, 4, 6 or 8: divisible by 2. Ends in 0 or 5: by 5. Ends in 0: by 10. Digit sum in the 3 or 9 table: the number is too.