Final answer:
To find the mass of the blue copper sulfate crystal, which is two-thirds the mass of the red fluorite crystal, use the equation m = (2/3)r, where m is the mass of the blue copper sulfate and r is the mass of the red fluorite crystal.
Step-by-step explanation:
The student is asked to write an equation to find the mass m (in grams) of the blue copper sulfate crystal, given that it is two-thirds the mass of the red fluorite crystal. We can represent the mass of the red fluorite crystal as r. Therefore, the equation to find the mass of the blue copper sulfate crystal would be m = (2/3)r.