All together, there are 5 rows. 150 divided by 5 is 30, so each row has 30 seats.
to make that a bit more visual for you, here:
(each 30 is a row and the underscore is the isle)
30 30 30___30 30
30 x 3 = 90
30 x 2 = 60
There are 90 seats on the left and 60 on the right.