additive counting rule
nounA rule that says if you can do one thing in some ways and another thing in other ways, you add the ways together to find the total.
Using the additive counting rule, if you can pick a red shirt in 3 ways or a blue shirt in 4 ways, there are 7 ways to pick a shirt.
OTHER MEANINGS
2. A basic counting principle stating that the number of ways to choose one item from either of two disjoint sets equals the sum of the sizes of the sets.
The additive counting rule tells us that choosing a card from a deck of hearts or a deck of spades gives 13 plus 13, or 26, choices.
3. A method in elementary arithmetic where quantities are combined by joining groups and counting the total.
Young children learn the additive counting rule by putting two groups of blocks together and counting all of them.
4. In probability, the rule that the probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.
By the additive counting rule, the chance of rolling a 2 or a 5 on a fair die is 1/6 plus 1/6, which is 1/3.