He can make 25 scarves from 15 balls of yarn.
Step-by-step explanation:
Given,
Yarn used for one scarf =
of a ball
Yarn purchased = 15 balls
Let,
x be the number of scarves.
Yarn used for one scarf*number of scarves = Yarn purchased

Multiplying both sides by 

He can make 25 scarves from 15 balls of yarn.
Keywords: fraction, multiplication
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