Dosage calculation practice by lesson

A common wrong answer — Divide by 1000

Dividing treats milligrams as the larger unit. It would turn 2 g into 0.002 mg, which describes a far smaller amount than the prescription actually states.

A common wrong answer — 200 mg is the amount in each millilitre

This reads the total as a per-millilitre strength. It would overstate the concentration fourfold, because 200 mg is spread across all 4 mL.

A common wrong answer — 200 mg is the dose wanted

This swaps the prescription for the label. Inverting the two produces the reciprocal of the correct volume — here 2.5 mL instead of 10 mL.

A common wrong answer — Multiplying 300 by 100

Multiplying gives 30 000, a number in no meaningful unit. A tablet count must be small enough to physically administer, so the size of the answer flags the error immediately.

A common wrong answer — The concentration of the available product

The concentration is needed later, to turn the dose into a volume. It cannot be used until the dose in milligrams exists.

A common wrong answer — Volume multiplied by time

Multiplying volume by time gives mL·hr, not mL/hr. Reading the unit before calculating rules this out immediately.

A common wrong answer — Multiply volume by rate

Multiplying gives a far larger number in no usable unit. A 500 mL bag at 100 mL/hr cannot take 50 000 hours.

A common wrong answer — It is always 20 gtt/mL

Sets differ, and using a remembered value instead of the supplied one changes the answer proportionally. Any drop factor must come from the question.

A common wrong answer — 40 mg

This multiplies by 100 rather than 1000 — one decimal place short. The metric step between grams and milligrams is three places, not two.

A common wrong answer — 800 mg/mL

This multiplies instead of divides. Spreading a fixed amount through a volume must make each millilitre weaker than the total, never stronger.

A common wrong answer — 2.5 mL

This divides 200 by 400 — the label by the prescription. The result is smaller than one dose of the available strength, which cannot be right when more than 200 mg is required.

A common wrong answer — 0.33 tablets

This divides the tablet strength by the dose instead of the other way round. The dose is larger than one tablet, so the answer has to be greater than 1.

A common wrong answer — 16 mg

This divides the weight by the rate. A dose per kilogram accumulates with weight, so a heavier patient must produce a larger dose, not a smaller one.

A common wrong answer — 9600 mL/hr

This multiplies instead of dividing. A rate spread over more hours must be smaller than the total volume, never larger.

A common wrong answer — 0.17 hours

This divides the rate by the volume. Ten minutes to deliver 900 mL at 150 mL/hr is impossible, so the size of the answer exposes the inversion.

A common wrong answer — 5 gtt/min

This divides the volume by the time and forgets the drop factor entirely, giving mL/min. The drop factor is what converts millilitres into drops.

A common wrong answer — 1.5 ÷ 500 = 0.003

This divides grams by milligrams without converting first. Dividing two different units produces a number that does not describe anything in the question.

A common wrong answer — 0.004 mg/mL

This divides 1 by 250 and labels the result in milligrams, skipping the gram-to-milligram conversion. The unit asked for decides when the conversion has to happen — before the division, not after.

A common wrong answer — 5.6 mL

This comes from rounding 0.625 to 0.7 before multiplying. Rounding an intermediate value carries the error into every step that follows — round only at the end.

A common wrong answer — 0.002 tablets

This divides 0.5 by 250 without converting grams to milligrams. Whenever a prescription and a label use different units, the conversion comes before the division.

A common wrong answer — 330 mg

This stops after stage one. The dose in milligrams is correct but the question asked for a volume, so the concentration step is still outstanding.

A common wrong answer — 3.3 mL/hr

This divides 150 by 45, producing millilitres per minute rather than per hour. Dividing by minutes always gives a per-minute rate, whatever unit the answer is labelled with.

A common wrong answer — 3 hours 75 minutes

This reads the digits after the decimal point as minutes. They are a fraction of an hour, so they must be multiplied by 60 — and 75 minutes is more than an hour, which cannot be a remainder.

A common wrong answer — 3000 gtt/min

This leaves the time in hours. Because the answer is per minute, the denominator has to be minutes — dividing by 5 instead of 300 overstates the rate sixtyfold.

A common wrong answer — Multiplying 250 micrograms by 74 kg

This treats a flat dose as a per-kilogram dose because a weight was supplied. A weight only enters the calculation when the prescription is actually written per kilogram.

A common wrong answer — Product B, because 1 g is more than 500 mg

This compares total amounts rather than concentrations. B holds more drug overall but spreads it through four times the volume, so each millilitre is weaker.

A common wrong answer — Multiplying by 62 kg first

A supplied weight invites a per-kilogram calculation even when the prescription is not written that way. Deciding what the question asks for before reading the numbers prevents this.

A common wrong answer — Rounding to 4 tablets

Rounding changes the dose from 175 mg to 200 mg. 3.5 is the mathematically correct result; whether the supplied dosage form can deliver it is a separate question, and rounding the number does not answer it.

A common wrong answer — 15 mL

This converts the whole daily total into a volume and never divides by three. When a question separates a daily total from a per-dose amount, the division into doses has to happen before the volume is worked out.

A common wrong answer — 500 mL/hr

This divides the full 1000 mL by the remaining 2 hours, ignoring the 500 mL already given. A rate change applies to what is left, not to the original total.

A common wrong answer — 12 hours 30 minutes

This divides the full 1000 mL rather than the 600 mL left. The question asks how much longer, so only the remaining volume belongs in the calculation.

A common wrong answer — Rounding 41.67 to 41

Truncating rather than rounding drops the value to the nearest whole number below. 41.67 is closer to 42, and rounding rules still apply at the final step.