Вся окружность =360°

1) 2/5*360=144°

144:2=72°

2) 4/15*360=96°

96:2=48°

3) 7/12*360=210°

210:2=105°

4)5/9*360=200°

200:2=100°

5)13/18*360=260°

260:2=130°

6)17/20*360=306°

306:2=153°

7)23/30*360=276°

276:2=138°