rule for 7. You multiply the final digit of the number by 4 and add the answer to the number preceding
it. If the answer is divisible by 13, then the number is divisible by 13. For example, let’s take 91. The
final digit is 1. We multiply 1 by 4 to get 4. We add 4 to the number in front of the 1, which is 9, to get
- And 13 is evenly divisible by 13, so 91 is as well.
Let’s try it again with 299. Is 299 evenly divisible by 13? It is hard to tell without actually doing a
division. We multiply the final digit, 9, by 4 to get an answer of 36. Add 36 to the 29 in front of the 9 to
get 65. You may recognise that 65 is 13 × 5, but if not, carry out the operation again. We are now
checking 65. Multiply 5 by 4 to get 20. Add 20 to the 6 in 65 to get 26. We can see that 26 is 2 × 13, so
there is no doubt now that 299 is evenly divisible by 13.
It is often useful to know if a number has factors that can be used to simplify a fraction or to simplify
a multiplication or division problem.