3.OA.B.5
Apply properties of operations as strategies to multiply and divide. Examples: If 6 × 4 = 24 is known, then 4 × 6 = 24 is also known. (Commutative property of multiplication.) 3 × 5 × 2 can be found by 3 × 5 = 15, then 15 × 2 = 30, or by 5 × 2 = 10, then 3 × 10 = 30. (Associative property of multiplication.) Knowing that 8 × 5 = 40 and 8 × 2 = 16, one can find 8 × 7 as 8 × (5 + 2) = (8 × 5) + (8 × 2) = 40 + 16 = 56. (Distributive property.)
Use multiplication facts to solve other multiplication problems. For example, use 6 * 3 = 18 to solve __ * 6 = 18 or 3 * 6 = __. Use the fact that multiplication is associative, to make a problem easier. For example, 7 * 2 * 5 can be evaluated by seeing 2 * 5 = 10, and 10 * 7 = 70, which is much easier.