Question 316167
First, we know that there must be 28 students in the class. This is because we know a grade received by 1/7 of the class, so the number of students must be divisible by 7. We also know the grade received by 1/4 and 1/2 of the class, so the number must be divisible by 4 and by 2. 28 is the only number less than 30 divisible by both 4 and 7. Feel free to check this. 

2 people got Bs. 
1/7, that is 4 people, got Cs. 
1/2, that is 14 people, got Ds. 
1/4, that is 7 people, got Fs. 

We know what grade was received by (2+4+14+7) = 27 people. This means that there is one person left to get the A.