The 10 ounce bottle costs $.1 an ounce because 1 / 10 = <<1/10=.1>>.1
The 16 ounce bottle costs $.125 an ounce because 2 / 12 = .125
The 25 ounce bottle costs $.125 an ounce because 2.5 / 20 = <<2.5/20=.125>>.125
The 50 ounce bottle costs $.1 an ounce because 5 / 50 = <<5/50=.1>>.1
The 200 ounce bottle cost $.05 an ounce because 10 / 200 = <<10/200=.05>>.05
The 200 ounce bottle is the cheapest per ounce because .05 < .08 < .1 < .125
He buys 1 bottle of ketchup because 10 / 10 = <<10/10=1>>1
#### 1