答案：C

解析：

Choice C is correct. Since xmL of the 50% solution will be combined with ymL of the 25% solution to obtain 300mL of a 40% solution, it follows that x+y=300.

Since the pharmacist will mix xmL of the 50% solution and ymL of the 25% solution, the amount of actual medicine in the mixture is 0.5x+0.25y.

The doctor orders 300mL of the medicine at 40% solution. Which is (300)*(0.4)=120 milliliters of medicine.

Since the amount of actual medicine in the mixture must equal the amount of actual medicine in the end result, 0.5x+0.25y=120.

This all gives the following system of equations:

x+y=300

0.5x+0.25y=120

To find the value of x, eliminate y. Multiplying the first equation by -0.25 gives the following system:

-0.25x-0.25y=-75

0.5x+0.25y=120

Adding the two equations yields: 0.25x=45 And so x=180.