Answer: John
Explanation:
Quincy = Cinnamon
Lucy = Pumpkin
Prudence = Nutmeg
John = GINGER
Since Quincy used the most, that means he'd have the least remaining ingrediant (cinnamon - 1/4 || .25). If Lucy used the least of her ingrediant, then that means she'd have the most remaining, which would be the pumpkin (5/8 || .625). And if Prudence used more of his ingredient than John, that means she'd have less ingrediant remaining, so between the nutmeg and the ginger, she clearly used the nutmeg (1/3 || .33). Which leaves John with the Ginger (1/2 || .5)
(I know this is one of those free diamond grab questions, so I already confirmed the answer when I got 100%)