Description for Correct answer:
Let number of students in examination halls P and Q is x and y, respectively.

Then, as per the first condition,

x - 10 = y + 10

= x - y = 20 ...(i)

As per the second condition,

\( \Large x + 20 = 2 \left(y - 20\right) \)

=> x + 20 = 2y - 40

=> x - 2y = -60 ...(ii)

On subtracting Eq. (ii) from Eq. (i), we get

-y + 2y = 20 + 60

=> y = 80

Putting the value of y in Eq. (i), we get

x - 80 = 20 => x =100

Hence, number of students in examination halls P and Q is 100 and 80, respectively.