В третьем классе 28 учеников.

Ответ:

1) (7/15-1/6) + 2/5 = 14/30-5/30+12/30=21/30=7/10

2)(3/8-1/9)+25/36 = 27/72-8/72+50/72=69/72

3)(8/9-5/6)+2/3 = 48/54-45/54+36/54=39/54

4)(7/8-13/20) + 9/10 = 35/40-26/40+36/40=45/40=1. 5/40= 1. 1/8

5)(7/18-1/12)+5/6 = 14/36-3/36+30/36=41/36=1. 5/36

6)(3/4-8/15)+17/20 =45/60-32/60+51/60=64/60=1. 4/60=1. 1/15