Probability is a chance that an event will occur and it lies b/w 0 to 1 (0-100%). For mutually exclusive events , probability is calculated by rule of addition P=P+P. For independent events, joint probability is calculated by rule of multiplication P=PxP. Given, probability of having a baby with bih weight >= 3000 grams; P= 0.5 probability of having a baby with bih weight 2500-2999 grams; P = 0.2 As these two events are mutually exclusive, probability of having a baby with Bw >2.5 kg; P is calculated by Rule of addition = 0.5+0.2 = 0.7