First you need to determine what operation needs to be performed based on the context of the problem. The list below describes a lot (but not all) of the types of problems you'll see
1. Combining 2 or more quantities? Addition
2. Adding on a quantity to some amount you already have? Addition
3. Finding the difference between two quantities? Subtraction
4. Taking one quantity away from another? Subtraction
5. Making some quantity of copies of another quantity? Multiplication
6. Repeatedly adding a number to itself some amount of times? Multiplication
7. Breaking a quantity into equally sized groups? Division
8. Repeatedly subtracting an equal quantity until you reach zero? Division
This problem seems most similar to type #8 above. You can imagine starting with 28 people and for each pizza, you can subtract off the 4 being fed by the pizza. You would repeatedly subtract 4 until all the people are fed (0 people remain).
To determine this value, you will find 28 / 4. If you remember your multiplication/division facts, this quantity is 7. If not, you can use the repeated subtraction method described above to determine how many 4'a are in 28.