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Terrance purchased a total of 10 pens and pencils for $4. Pens cost 50 cents and pencils cost 25 cents. Terrance wrote the system of equations below to represent the situation.

{ x + y = 10
{ 1/2x + 1/4y = 4

A: What units are associated with the expressions in the second equation?

B: Describe the constraint that each equation places on the system.

C: Are there any other constraints on the system? If so, what are they?