It is important to note that all the information's required are already given in the question
Probability that a day will be hot = 0.15 + 0.10 = 0.25 Probability that a day will be sunny = 0.15 + 0.30 + 0.10 = 0.55 Adding the above two probabilities, we get = 0.25 + 0.55 = 0.80 Now Probability that a day will be both hot and sunny = 0.15 So Probability that a day was hot or sunny = 0.80 - 0.15 = 0.65 I hope the procedure is clear enough for you to understand.
it is subtraction because it is asking how many more pounds of gravel does Ben use in his front yard than his back yard and his front yard has a bigger number so you subtract.