The best thing to do is to find out how many cans can fed four people (as 24 and 28 are both multiples of 4)
To do this, divide 96 by 24/ 96/24= 4
Therefore, 1 can can feed 4 people.
As you've got 28 people, you need to divide 28 by 4.
28/4= 7
Therefore, Ann should by 7 cans to feed 28 people
Hope this helps :)