What is the least five-digit positive integer which is congruent to 5 (mod 15)?
Well the number should be in the form 15x+5 where x is any integer. It has to be the least five digit positive integer. The smallest one is 10000 so we form an inequality.
We round 666 1/3 up to 667. Substtute 667 into 15x+5 to get 10010.