a)
1 + (20 – 2) = 1 + 18 = 19
10 + (20 – 12) = 10 + 8 = 18
б)
17 – 8 + 2 = 9 + 2 = 11
15 – 12 + 9 = 3 + 9 = 12
13 + (18 – 15) = 13 + 3 = 16
15 – (17 – 16) = 15 – 1 = 14
Ответ: a) 19; 18; б) 11; 12; 16; 14