To solve this problem it is simpler to consider the negation of the probabilities - that is, the probability that it will not rain on either Saturday or Sunday. The probabilities of no rain on Saturday and Sunday are $$ P(\text{no rain on Sat}) = 1 - 0.6 = 0.4, $$ $$ P(\text{no rain on Sun}) = 1 - 0.7 = 0.3. $$ The probability that it will not rain on the weekend is given by $$ P(\text{no rain on weekend}) = P(\text{no rain on Sat})P(\text{no rain on Sun}) $$ which is $$ P(\text{no rain on weekend}) = 0.12 $$ So, the probability of it raining on the weekend is $$ P(\text{rain on weekend}) = 1 - P(\text{no rain on weekend}) = 0.88 $$ or $88\%$.