GATE CS 2017 Set 2 QGA-4 — General Aptitude

Let $x$ be the number of multiple-choice questions and $y$ be the number of essay questions.

According to the problem, we can set up the following system of linear equations:

1. Total number of questions:
   $$x + y = 20 \quad \text{--- (Equation 1)}$$
2. Total marks:
   $$3x + 11y = 100 \quad \text{--- (Equation 2)}$$

From Equation 1, we can express $y$ in terms of $x$:

Substituting this value of $y$ into Equation 2:

Thus, the exam has 15 multiple-choice questions.

Therefore, the correct option is B.