He can buy 6 small bags because 24 / 4 = <<24/4=6>>6
These give him 300 balloons, because 6 x 50 = <<6*50=300>>300
He can buy 4 medium bags because 24 / 6 = <<24/6=4>>4
These give him 300 balloons because 4 x 75 = <<4*75=300>>300
He can buy 2 extra large bags because 24 / 12 = <<24/12=2>>2
This gives him 400 balloons because 2 x 200 = <<2*200=400>>400
400 is the most he can buy because 400 is greater than 300
#### 400