#### Answer

The patient’s percent decrease in his blood glucose level is 40%.

#### Work Step by Step

To find the percent change, we use the following formula:
percent change = $\frac{New\ amount\ -\ Original\ amount}{Original\ amount}$ $\times$ 100%
The original blood glucose level was 300 mg/dL. The new blood glucose level is 180 mg/dL.
Substituting these values into the formula yields:
percent change = $\frac{180-300}{300}$ $\times$ 100%
percent change = $\frac{-120}{300}$ $\times$ 100%
percent change = -0.4 $\times$ 100%
percent change = -40%
The patient’s percent decrease in his blood glucose level is 40%.