It depends on the question asked and which operation you are doing.
The operation you need to do depends on the probability question asked.
If there is an indication of several outcomes combined, the probabilities are multiplied. This is indicated by "AND"
Probability that the the wind is blowing
Three probabilities would be multiplied together - that means the numerators and the denominators.
If there is an indication of a choice, then the probabilities have to be added. This is indicated by OR
Probability that Anne takes a pear
In this case the probabilities are added, which means you need to find a common denominator, make equivalent fractions, and then only add the numerators, keeping the same denominator.