An order is written for 500 mL of NS to infuse at 40 gtt/min. How long would it take to infuse the NS if the set delivers 10 gtt/mL?

10 gtt/mL means 10 drops per mL. So for 500 mL you will get:
10 x 500 = 5,000 drops
The flow rate is 40 gtt/min, which means 40 drops per minute, so divide total drops by 40 to get the number of minutes:
5,000/40 = 125