# Concept derivation guides

299 concept teaching guides, separate from the 36,000 individual question derivations.

[Search all questions & derivations →](/en/explore/)

1 1 results · Page 1 of 1



Concept guide



## [Basic counting principles: how many three-digit even numbers have distinct digits](/en/derivations/derivation-mko-counting-principle-01/?from=%2Fen%2Fderivations%2F%3Fq%3Dmko-counting-principle)

Count the even integers from 100 to 999 whose three digits are pairwise distinct. Requirements: add only disjoint classes, and before each multiplication confirm that the number of options at every stage is the same for all earlier choices; when it is not, branch first.

Basic counting principles

derivation-mko-counting-principle-01





1 1 results · Page 1 of 1