need to consider 2 cases. Answer is not simply P(Bus A is late) + P(Bus B is late) since P(Bus A is late) doesn’t consider the cases where B is involved (meaning that P(Bus A is late) is the probability of Bus A being late regardless of bus B, but the qn asked to consider specific cases of EXACTLY one of the buses being late i.e. if Bus A is late, Bus B has to be punctual and vice versa.)