Дробь к общему знаменателю: : 1/30 и 1/50 , 1/24 и 1/120 , 3/40 и 7/25 , 5/12 и 7/8 , 2/55 и 5/22 , 1/48 и 1/72 , 9/35 и 11/42.

1)
1/30 и 1/50
30=2·3·5
50=2·5·5
Н.О.З(30;50)=2·3·5·5=150
1/30=5/150
1/50=3/150
2)
1/24 и 1/20
24=2·2·2·3
20=2·2·5
Н.О.З(24;20)=2·2·2·3·5=120
1/24=5/120
1/20=6/120
3)
3/40 и 7/25
40=2·2·2·5
25=5·5
Н.О.З(40;25)=2·2·2·5·5=200
3/40=15/200
7/25=56/200
4)
5/12 и 7/8
12=2·2·3
8=2·2·2
Н.О.З.(12;8)=2·2·3·2=24
5/12=10/24
7/8=21/24
5)
2/55 и 5/22
55=5·11
22=2·11
Н.О.З.(55;22)=5·11·2=110
2/55=4/110
5/22=25/110
6)
1/48 и 1/72
48=2·2·2·2·3
72=2·2·2·3·3
Н.О.З.(48;72)=2·2·2·2·3·3=144
1/48=3/144
1/72=2/144
7)
9/35 и 11/42
35=5·7
42=2·3·7
Н.О.З.(35;42)=5·7·2·3=210
9/35=54/210
11/42=55/210