Common Registration Assessment, Part 1
Fewest tablets to make a dose
Three original questions on combining tablet strengths, with the working shown and the usual traps pointed out.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
These questions hand you a prescribed dose and a list of available strengths, then ask for the smallest number of whole tablets that makes the dose exactly. It is a supply question, not a dosing decision.
Start with the largest strength and work down, checking that the remainder can be made exactly. If the remainder cannot be made, drop one large tablet and try again.
The method
- Write down the dose and every available strength.
- Take as many of the largest strength as fit without overshooting.
- Check the remainder can be made exactly with smaller strengths.
- If not, remove one large tablet and rebuild the remainder.
- Count the total tablets and confirm the strengths add back to the dose.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Drug X is prescribed at 175 micrograms daily. Tablets of 25 micrograms and 100 micrograms are available. What is the fewest number of tablets that makes the dose?
Show the answer
Answer: 4 tablets: one 100 micrograms and three 25 micrograms
Working: 175 = 100 + 25 + 25 + 25. Using seven 25 microgram tablets also works but needs 7 tablets, so the fewest is 4.
Question 2
Drug Y is prescribed at 6 mg daily. Tablets of 1 mg, 3 mg and 5 mg are available. What is the fewest number of tablets?
Show the answer
Answer: 2 tablets: one 5 mg and one 1 mg
Working: 5 + 1 = 6 mg in 2 tablets. Two 3 mg tablets also gives 6 mg in 2 tablets; six 1 mg tablets takes 6, so 2 is the fewest.
Question 3
Drug Z is prescribed at 17.5 mg daily. Tablets of 2.5 mg and 5 mg are available. What is the fewest number of whole tablets?
Show the answer
Answer: 4 tablets: three 5 mg and one 2.5 mg
Working: 5 + 5 + 5 + 2.5 = 17.5 mg in 4 tablets. Using 2.5 mg tablets alone needs 7, so 4 is the fewest.
How this is examined
The trap is greediness: grabbing the biggest strength twice and overshooting. Work down from the largest strength, and always count your tablets twice.
Where marks are lost
- Stopping at the first combination that works instead of the one with fewest tablets.
- Splitting tablets when the question asks for whole tablets only.
- Overshooting the dose with the largest strength and not adjusting.
Frequently asked questions
Can I ever halve a tablet in these questions?
Only if the stem says the tablets are scored and may be halved. Otherwise assume whole tablets only.
Is there always a unique answer?
Not always. Two different combinations can use the same fewest number of tablets, as in the 6 mg example above. State the combination you chose and count carefully.
Sources
- BNF, guidance on prescribing
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Oral maintenance dose and dosing interval
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
- Unit conversions for mass and volume
- Units and units per mL
- Reconstituting powders and final concentration
- Millimoles and milliequivalents
- Infusion time and volume remaining
- Days supply from pack sizes
- Percentage w/v and w/w conversions
- Ideal and adjusted body weight from formulae
- Diluting creams and ointments by weight
- Scaling a formula to a batch size
- Age-band doses from a provided table
- Tablet fractions and split doses
- Stones, pounds, inches and metric conversions
- Micrograms per minute from mg per hour
- Percentage change and mark-up
- Molar solutions and mass per volume
- Osmolarity from component concentrations
- Direct proportion and cross-multiplication
- Scientific notation and very small quantities
- Stock use and reorder quantities
- Parts per million and very dilute solutions
- Sodium chloride equivalents and isotonic solutions
- Density and specific gravity conversions
- Doses per day and hours between doses
- Finding the volume of diluent to add
- Capping a dose at a stated maximum
- Whole vials and ampoules to draw up
- Reading values from a provided graph
- Micrograms per kg per minute infusions
- Percentage composition of a compound preparation
- Suppository displacement values
- Isotonicity by freezing point depression
- Energy content of parenteral nutrition
- Syringe driver rate calculations
- Area under the curve calculations
- Renal dose adjustments using provided bands
- Insulin units and infusion calculations
- Working backwards from a stated dose
- Doses and days from multi-dose devices
- Combined additives in one infusion
- BMI calculations for the GPhC assessment
- Clearance from half-life and volume of distribution
- Maintenance fluid requirements by weight
- Sodium load from effervescent tablets
- Ethanol content of liquid preparations
- Oxygen cylinder duration calculations
- Urine output and fluid balance calculations
- Quantities for titration packs
- Infusion line priming and dead space
See also the formula sheet, the eight sample questions and the approved calculator page.