At a certain school, each of the 150 students takes between 1 and 3 classes

At a certain school, each of the 150 students takes between 1 and 3 classes. The 3 classes available are Math, Chemistry and English. 53 students study math, 88 study chemistry and 58 study english. If 6 students take all 3 classes, how many take exactly 2 classes?

In this case, we'd use the first formula, since we want the number who take exactly 2 classes:

150 = 53 + 88 + 58 - (doubles) - 2(triples)
150 = 199 - (doubles) - 2(6)
150 = 187 - doubles
doubles = 37

Let's just change the question a tiny bit: 

At a certain school, each of the 150 students takes between 1 and 3 classes. The 3 classes available are Math, Chemistry and English. 53 students study math, 88 study chemistry and 58 study english. If 6 students take all 3 classes, how many take at least 2 classes?

 In this case, we'd use the second formula, since we want the number who take at least 2 classes:

150 = 53 + 88 + 58 - (at least 2 of the 3) + (all 3)
150 = 199 - (at least 2 of 3) + 6
150 = 193 - (at least 2 of 3)
At least 2 of 3 = 43