thesaurus.maths.org

The expression 5 ×(3+6) could be calculated in two ways. One way of calculating it is 5 ×9, working out the brackets first, and giving the answer 45.
Another way of calculating it could be (5 ×3)+(5 ×6) i.e multiplying everything inside the brackets by 5, which gives 15+30, an answer of 45.
This demosntrates the fact that multiplication is distributive over addition.

An operation ⊗ is distributive over another operation ⊕, if A⊗(B ⊕C)=(A ⊗B) ⊕(A ⊗C) for any choice of A, B and C.
For instance, multiplication is distributive over addition: A(B+C)=AB+AC but addition is not distributive over multiplication: A+(BC) is not the same as (A+B)(A+C).