The number of seats in the first row of a concert hall is 6. th second row has 9 seats, the third row has 12 seats, and the fourth row has 15 seats. how many seats will be in the eighth row?

Eighth row has 27 seats

It's a pattern, you add three to every row. (1st row) 6+3=9, (second row) 9+3=12, (third row) 12+3=15, (fourth row) 15+3=18, (fifth row) 18+3=21, (sixth row) 21+3=24, (seventh row) 24+3=27 (eighth row) 27+3=30.