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Answer:
 a) 80 g fish
 b) 200 g butter
Step-by-step explanation:
The recipe is multiplied by ...
 (number served)/(number recipe serves)
to find the amount of ingredients for a different number of people.
__
 fish for 2 people = (2/6)·(240 g) = 80 g
__
 butter for 15 people = (15/6)(80 g) = 200 g