Mark use 1pt. 9fl. oz. them 2pt. 10 fl oz from a container of film developer that hold 3qt. How much of the developer remains?
Add:
1 pt. 9 oz + 2 pt. 10 oz = 3 pt. 19 oz. = 4 pt. 3 oz.
Since there are 2 pints in a quart, Mark originally started with 6 pints. Change that to 5 pints 16 oz.
Subtract.