Question

There are some students in the 2 examination halls A and B. to make the number of students equal in each hall, 10 students are sent from A to B. but if 20 students are sent from B to A, the number of students in A becomes double the number of students in B. find he number of students in each hall.

Answer 1

A=B+20 ------->I
A+20=2(B-20)
A=2B-60------>II
using substitution method
2B-60=B+20
B=20+60=80
A=B+20
Therefore A=100
Therefore there are 80 students in hall B and 100 students in hall A.

Note* Some students maybe thinking how A=B+20
For them:
A-10=B+10
A=B+20