First subtract 4 from 7 since they are just asking for the amount he paid for pitches. Then, take the difference (3) and divide it by .75 to get your answer (4 groups).
It really depends on what kind of problem. like if you are doing a word problem or whatever than you should read the question a couple times, underline key words, DECIDE which operation to use, refer to the question and see if it makes sense. Hope I helped :)