Final answer:
A prescription with 120 tablets of Famotidine 20mg, with a dosage of 40mg BID, will last for 30 days, as the patient will take four tablets daily.
Step-by-step explanation:
The question asks how many days a prescription of Famotidine 20mg tablets will last when the patient is instructed to take 40mg orally twice daily (BID). Given the prescription has a total of 120 tablets of 20mg each, and the prescribed dosage is 40mg BID, the patient will take two 20mg tablets per dose, which amounts to four tablets per day. To calculate the duration, the total tablets (120) are divided by the daily usage (4 tablets), resulting in 30 days of medication.