If a and b are mutually exclusive, the maximum value of the sum S=P(a) + P(b) cannot exceed 1. Since 0.48+0.62=1.1 >1, a and b cannot be mutually exclusive.
If it is still not clear, sketch a Venn diagram to convince yourself that it is not possible to draw a and b such that P(a)+P(b)>1 while a and b are mutually exclusive.
