This question tests percent decrease, a common NEX topic. First find the amount of decrease: 50 mg # 40 mg
= 10 mg. Next divide the decrease by the original amount:
10 ÷ 50 = 0.2.
Convert the decimal to a percent by multiplying by 100:
0.2 × 100 = 20%.
Therefore, the correct answer is B. Option D represents the numerical decrease without considering the original value. Option A and C result from incorrect division. On the NEX exam, percent change problems always use the original value as the denominator.