Ответ:

15) \( 1 \frac{3}{4} + \frac{9}{10} = 1 \frac{3 \cdot 5}{4 \cdot 5} + \frac{9 \cdot 2}{10 \cdot 2} = 1 \frac{15}{20} + \frac{18}{20} = 1 + \frac{15+18}{20} = 1 \frac{33}{20} = 1 + 1 \frac{13}{20} = 2 \frac{13}{20} \)