Помогите решить примеры:

19/42-5/63

(7/10+3/5) - (1/6+2/3)

2/3 - (1/20+2/9) - 5/18

1/3+1/4+1/5+1/6+1/7

19/42-5/63 = (57-10) 126=47/126

(7/10+3/5) - (1/6+2/3) = 7/15

7/10+3/5 = (7+6) / 10=13/10

1/6+2/3 = (1+4) / 6=5/6

13/10-5/6 = (39-25) / 30=14/30=7/15

2/3 - (1/20+2/9) - 5/18=7/60

1/20+2/9 = (9+40) / 180=49/180

2/3-49/180 = (120-49) / 180=71/180

71/180-5/18 = (71-50) / 180=21/180=7/60

1/3+1/4+1/5+1/6+1/7 = (140+105+84+70+60) / 420=459/420=1 39/420