20. \(\frac{17}{90}+\frac{19}{120}\)

НОК(90, 120) = 360. \(\frac{17}{90}+\frac{19}{120}=\frac{68}{360}+\frac{57}{360}=\frac{125}{360}=\frac{25}{72}\).

Ответ: \(\frac{25}{72}\).