a.
420:42 - 40:5 = 10 - 8 = 2
420 : (42 - 40) : 5 = 420 : 2 : 5 = 210 : 5 = 42
b.
218 - 68 : 2 x 0 = 218 - 34 x 0 = 218 - 0 = 218
(218 - 68) : 2 x 0 = 150 : 2 x 0 = 75 x 0 = 0
c.
200 - 100 : 100 - 100 = 200 - 1 - 100 = 199 - 100 = 99
(200 - 100) : 100 - 100 = 100 : 100 - 100 = 1 - 100 = - 99