a) 3,67-0,39-4,5 = 3.67 - (0.39+4.5) = 3.67 - 4.89 = - 1.22

б) 13,81-20,7-(-8,1) = 13.81 - 20.7 + 8.1 = 21.91 - 20.7 = 1.21