We can write:
Quantity A: (0.4)(250)
Quantity B: (0.8)(125)

Before you resort to using the GRE's onscreen calculator, you might see that we can quickly answer this question using a technique called "Multiplying by Doubling and Halving"
The idea with this premise that: AB = (2A)(B/2)
For example: (4.5)(6) = (9)(3) [we doubled the 4.5 and we halved the 6]

Using this same technique, we can see that (0.4)(250) = (0.8)(125) [we doubled the 0.4 and we halved the 250]

We now have:
Quantity A: (0.8)(125)
Quantity B: (0.8)(125)

Answer: C

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