Trey evaluated 12/15 + 1/10 and got an answer of 13/25. Which statement is true about his answer?

You haven't posted the answer choices, but Trey got the wrong answer.

The denominators must be the same in order to add the fractions. In this case, the denominators 15 and 10 are different. We cannot add in this current state.

Adding the denominators is a common mistake which you should practice to avoid.