Answer: The nurse needs 40 minutes to infuse the dose at a 20% faster rate.
Explanation:
In order to calculate how much time the nurse needs to infuse the dose, you have to calculate how much is the 20%
The 20% of 50 minutes is obtained multiplying 50 by the factor that represents the 20% which is

= 10 minutes
Now, the time she needs to infuse the dose at a faster rate is obtained as follows:
50 minutes - 10 minutes = 40 minutes.
Notice that you have to subtract the 20% of 50 minutes to the original time to obtain a 20% faster rate.