Ответ:

1) -12+15-6+12= -12+12+15-6=9 ( -12+12 = 0)

2) 16+(-20) + 14 + (-10) = 16-20 + 14 -10 = 16+14 - 20 - 10 = 0 ( т.к. 16+14=30, -20-10=-30, -30+30 = 0)

3) 13+(-9)+(-17) + 19 = 13-9-17+19= 13+19-9-17 = 32 - 16 =16

4) 0,75 + (-2,9) + (-1,05) + (-0,75) = 0,75 - 0,75 - 2,9 - 1,05 = - 4, 05

5) 1/4 + (-1 1/4) + 4 3/8 + (-6 1/4) =  1/4 -1 1/4 + 4 3/8 -6 1/4 = 1/4 - 1 1/4 = -1 - 6 1/4 = - 7 1/4 + 4 3/8 = - 3 1/2