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:

Explanation:-

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.