a =(250-25)*4-(15 x 5 + 35 x 6) = 225 x 4 - (75 + 210) = 900 - 285 = 615
b = 15 x (3 x 4 + 8/(5 x 2 - 6) x 2) = 15 x (12 + 8/(10-6) x 2) = 15 x (12 + 8/4 x 2) = 15 x 12 + 2 x 2) = 15 x (12 + 4) = 15 x 16 = 240
c = (3,5 x (10-3) + 3/5)/(12,5 - 5/2) = (3,5 x 7 + 0,6)/(12,5 - 2,5) = (24,5 + 0,6)/10 = 25,1/10 = 2,51