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A child weighs 18 kg. 'Drug X' (a fictional medicine for this exercise) is prescribed at 15 mg/kg per dose, up to four times a day, with a maximum of 60 mg/kg per day. It comes as a liquid containing 120 mg in 5 mL. (1) What dose in mg should the child receive? (2) What volume in mL? (3) What is the maximum total daily dose? (4) The parent says they have already given 1,000 mg today. Can they give another dose? You have 5 minutes and no calculator.
Say your answer out loud first, then jot down the key points you made. Writing it helps you spot gaps — and it's what the AI feedback reads.
0 words
Accurate, calm arithmetic with units, checking your answer, and applying it safely — the kind of thinking behind safe prescribing.
A structure to build your own answer on — not a script.
Write down what you know
Weight 18 kg; 15 mg/kg per dose; max 60 mg/kg/day; concentration 120 mg/5 mL.
Dose
18 × 15 = 270 mg.
Volume
120 mg in 5 mL → 24 mg/mL; 270 ÷ 24 = 11.25 mL.
Daily maximum
18 × 60 = 1,080 mg per day.
Apply safely
1,000 + 270 = 1,270 mg > 1,080 mg: do not give another full dose; seek advice.
Sanity check
Does 11.25 mL seem reasonable? Re-check units and arithmetic aloud.
A strong but not perfect answer. Read the notes to see why it works — then make your own version with your experiences.
I'll write down what I know first1: the child weighs 18 kg, the dose is 15 mg per kg, the maximum is 60 mg per kg per day, and the liquid is 120 mg in 5 mL.
First, the dose: 18 times 15. 10 times 15 is 150, 8 times 15 is 1202, so 270 mg per dose.
Second, the volume. 120 mg in 5 mL means 24 mg in each mL. So I need 270 divided by 24. 24 times 11 is 264, which leaves 6, and 6 is a quarter of 24 — so 11.25 mL. To check: 11.25 times 243 is 264 plus 6, which is 270. Good.
Third, the maximum daily dose is 18 times 60, which is 1,080 mg.
Finally, the parent has already given 1,000 mg. Another full dose would take the total to 1,270 mg, which is well over the 1,080 mg maximum. So they shouldn't give another dose. Only 80 mg remains within the limit, and in reality you wouldn't give an odd partial dose without advice — I'd tell them not to give any more for now and to speak to a pharmacist or doctor. I'd also want to check exactly what was given and when4, in case they've made an error or used a different product.
In real life I'd always double-check a calculation like this with a colleague, because small errors with children can be serious.
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