Final answer:
The missing number in the equivalent expressions 15+9=? (5+3) is 3. We use the distributive property to solve for the unknown number, resulting in the equivalent expressions 15+9=3(5+3).
Step-by-step explanation:
To find the missing number in the pair of equivalent expressions, we use the distributive property which in this case simplifies 15+9 to the equivalent of ? (5+3). We need to find a number that when multiplied by the sum of 5 and 3 will give us the sum of 15 and 9, which is 24.
So, we have ? × (5+3) = 15+9, which leads to ? × 8 = 24. To find the missing number, which we will call x, we divide both sides of the equation by 8, resulting in x = 24 ÷ 8, which simplifies to x = 3.
Therefore, the missing number in the pair of equivalent expressions is 3, making the two expressions equivalent: 15+9 = 3(5+3).