There are two examinations rooms A and B. If 10 students are sent from A to B, then the number of students in each room is the same. If 20 candidates are sent from B to A, then the number of students in A is double the number of students in B. The number of students in room A is:

Answer: Option C

Let the number of students in rooms A and B be x and y respectively.
Then,
x - 10 = y + 10
⇒ x - y = 20 . . . . . (i)
and
x + 20 = 2(y - 20)
⇒ x - 2y = -60 . . . . . (ii)
Solving (i) and (ii) we get:
x = 100, y = 80
∴ The required answer A = 100