We can solve this question using the formula: Average = Sum/No. of values
33 = Sum of marks of 120 candidates/120
Sum of marks of 120 candidates (Let this be S1) = 3960 (i.e. 33*120) ----- Equation 1
Let the no. of candidates who passed = X
Thus, no. of candidates who failed will be = 120 - X
We know that the average marks of candidates who passed = 37
Using the same average formula we can say that,
37 = Sum of candidates who passed (Let this be SP) /X
37X = SP ---- Equation 2
We know that the average marks of candidates who failed = 13
Using the same average formula we can say that,
13 = Sum of candidates who failed (Let this be SF) / (120-X)
13(120-X) = SF ----- Equation 3
Further, Sum of marks of 120 candidates = Sum of candidates who passed + Sum of candidates who failed, i.e. S1 = SP + SF
From equation 1, 2 and 3 we can say,
37X + 13(120-X) = 3960 24X = 2400
X = 100
The number of candidates that passed = 100
Answer choice (A)