Since you know there are a total of 912 chairs in total, you have to figure out how many blue chairs are there.
In order to find the amount of yellow chairs, you have to figure out how many blue chairs are there.
First: 12 chairs in each row x 36 rows = 432.
So you now know that there are 432 blue chairs.
Next: 912 - 432 = 480.
There are a total of 480 yellow chairs.
STOP HERE IF YOU NEEDED TO KNOW HOW MANY YELLOW CHAIRS THERE ARE IN TOTAL!
Now we know that there are 480 yellow chairs. To find out how many rows there are you need to divide.
480 / 20 = 24.
So there are 24 rows of yellow chairs.
Hoped this helped,
-Anime
