Answer: Hello!
You know that 4 out of 28 had to take summer school, and you want to know the ratio of students who did not have to take summer school.
If 4 out of 28 had to take it, then 28 - 4 = 24 did not have to take sumer school.
then the ratio for 24 out of 28, is 24/28
Now we want to express it in it's lowest therms.
if we divide both sides of our division by 4, we got that 24/4 = 6 and 28/4 = 7, then our ratio 24/28 can be writen as 6/7.