4
votes
4answers
1k views

divisibility for numbers like 13,17 and 19 - Compartmentalization method

For denominators like 13, 17 i often see my professor use a method to test whether a given number is divisible or not. The method is not the following : Ex for 17 : subtract 5 times the last digit ...
7
votes
3answers
387 views

Number of integers not divisible by $p$ and $q$

Here's a part of question from Siklos' "Advanced Problems in Core Mathematics": How many integers greater than or equal to zero and less than 1000 are not divisible by 2 or 5? What is the average ...