To add these two mixed numbers, we first add their whole number parts and then their fractional parts separately.
Whole number parts: 10 + 13 = 23
Fractional parts: 9/12 + 9/12 = 18/12
We can simplify 18/12 by dividing both the numerator and denominator by their greatest common factor, which is 6.
18/12 = (18 ÷ 6) / (12 ÷ 6) = 3/2
Therefore, the sum of 10 9/12 and 13 9/12 is:
23 3/2 or 23 and 1/2.