Explanation:
Let there be x row is first hall
then in 2nd hall number of rows will be = x+5
Let there be y seats in each row of the first hall
then, there would be y+10 seats in each row of the 2nd hall.
Therefore, according to question
(y+10)(x+5) =480.....(i)
and xy = 420.......(ii)
Solve these two equations you will get the required number of seats in each row of the first hall have.