10. Вычислите: 1) 15/20 + 42/30 − 56/40; 2) (1/2 − 1/3) + (1/3 − 1/4); 3) 17/51 + 19/57 − 13/39; 4) 2/3 + 1 1/2 − 1/4; 5) (3/6 − 1/4) + (1/2 − 1/3); 6) (1 3/5 − 3/10) + (1/4 − 1/8).

1. \(\frac{15}{20}+\frac{42}{30}-\frac{56}{40}=\frac34+\frac75-\frac75=\frac34\).
2. \(\left(\frac12-\frac13\right)+\left(\frac13-\frac14\right)=\frac12-\frac14=\frac14\).
3. \(\frac{17}{51}+\frac{19}{57}-\frac{13}{39}=\frac13+\frac13-\frac13=\frac13\).
4. \(\frac23+1\frac12-\frac14=\frac23+\frac32-\frac14=\frac{8+18-3}{12}=\frac{23}{12}=1\frac{11}{12}\).
5. \(\left(\frac36-\frac14\right)+\left(\frac12-\frac13\right)=\left(\frac12-\frac14\right)+\frac16=\frac14+\frac16=\frac5{12}\).
6. \(\left(1\frac35-\frac3{10}\right)+\left(\frac14-\frac18\right)=\left(\frac85-\frac3{10}\right)+\frac18=\frac{13}{10}+\frac18=\frac{57}{40}=1\frac{17}{40}\).

Ответ: 1) \(\frac34\); 2) \(\frac14\); 3) \(\frac13\); 4) \(1\frac{11}{12}\); 5) \(\frac5{12}\); 6) \(1\frac{17}{40}\).