🧠 The theory: everything hinges on the daily amount
A supply question is a rate problem. Once you know the amount used per day, a total is just the daily amount times the number of days, and a duration is just the total quantity divided by the daily amount. The two directions are the same relationship rearranged, which is why finding the amount per day first always unlocks the answer.
Worked example
A patient takes 10 mL three times a day for 28 days. What total volume is needed?
- Step 1 — Find the amount used in one day: 10 mL x 3 doses = 30 mL a day.
- Step 2 — Multiply by the number of days: 30 x 28 = 840.
Answer: 840 mL
Tip: You can only dispense whole packs, so round up to the next bottle size, for example 900 mL.
Worked example
How many days will a 200 mL bottle last at 5 mL four times a day?
- Step 1 — Find the amount used per day: 5 mL x 4 = 20 mL a day.
- Step 2 — Divide the total by the daily amount: 200 / 20 = 10.
Answer: 10 days
Worked example
How many tablets are needed for one tablet twice a day for 28 days?
- Step 1 — Per day: 1 x 2 = 2 tablets.
- Step 2 — Over the course: 2 x 28 = 56.
Answer: 56 tablets
Creams, inhalers and patches
Worked example
A cream is applied twice a day, using about 1 g each time, for 30 days. How much should be supplied?
- Step 1 — Amount per day: 1 g x 2 = 2 g.
- Step 2 — Over 30 days: 2 x 30 = 60.
Answer: 60 g (for example two 30 g tubes)
Worked example
A 200-dose inhaler is used as 2 puffs twice a day. How long will it last?
- Step 1 — Puffs used per day: 2 x 2 = 4 puffs.
- Step 2 — Divide the doses in the inhaler by the daily use: 200 / 4 = 50.
Answer: 50 days
Harder: eye drops
Worked example
One drop into each eye twice a day. A 10 mL bottle delivers about 20 drops per mL. How long, in theory, would the bottle last?
- Step 1 — Drops used per day: 1 drop x 2 eyes x 2 times = 4 drops.
- Step 2 — Drops in the bottle: 10 mL x 20 drops/mL = 200 drops.
- Step 3 — Divide: 200 / 4 = 50 days in theory.
Answer: 50 days in theory
⚠️ Watch the in-use shelf life
Most eye drops must be discarded 4 weeks (28 days) after opening, regardless of how much is left. So although the bottle above could last 50 days mathematically, in practice it is discarded at 28 days. The exam likes this distinction.
Test yourself
5 mL three times a day for 7 days. What volume is needed?
Answer: Per day = 15 mL; total = 15 x 7 = 105 mL (round up to a 150 mL bottle)
A 120-dose inhaler used as 2 puffs four times a day. How long does it last?
Answer: Per day = 8 puffs; 120 / 8 = 15 days
How long will 300 mL last at 10 mL twice a day?
Answer: Per day = 20 mL; 300 / 20 = 15 days
One tablet three times a day for 28 days. How many tablets?
Answer: Per day = 3; total = 3 x 28 = 84 tablets