To find the missing denominator in the addition sentence \( \frac{15}{28} + \frac{9}{?} \), we need the fractions to have a common denominator.
The denominator in the first fraction is 28. Therefore, we should convert the second fraction so that its denominator is also 28.
To find the missing denominator (let’s call it \( x \)), we can set it equal to 28:
So, the addition can be represented as: \[ \frac{15}{28} + \frac{9}{28} \]
Thus, the missing denominator is 28.
The answer is: 28