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Can you help your pre-reg answer these revision calculations?

Part one of the GPhC pre-reg assessment is made up of 40 calculation questions
Part one of the GPhC pre-reg assessment is made up of 40 calculation questions

Are you able to help your pre-reg with these revision calculations?

Your pre-reg has their GPhC registration exam this week, so you decide to give them two calculation questions to complete. After checking their answers, you inform them that unfortunately they have got both questions wrong.

Can you spot their errors and answer these questions correctly?

Question one: potassium permanganate strength

A patient is directed to use 40ml of a 1 in 8,000 solution of potassium permanganate each day for one week. The prescription directs you to make a solution 10 times this strength.  

How much potassium permanganate, in micrograms, will be required to dispense this prescription?

Here is the pre-reg’s working out:

40ml x 7 days (one week) = 280ml of 1 in 8,000

10 times stronger, so must be 1 in 800 

So 28ml of 1 in 800

1 in 800 = 0.00125%

0.00125g in 100ml

So 0.00035g in 28ml

= 0.35 mg

This answer is incorrect. What answer did you get?

Click here for the correct working out and answer

One method of working out the answer:

40ml x 7 days (one week) = 280ml of 1 in 8,000

= 1g in 8,000ml

= 1,000mg in 8,000ml

= 1mg in 8ml

= 35mg in 280ml

= 35,000mcg in 280ml

An alternative method for working out the answer is:

1 in 8,000 = 1g in 8,000ml = 0.0125g in 100ml = 0.0125%w/v

C1 x V1 = C2 x V2 (C1 = 0.125%w/v; V1 = ?; C2 = 0.0125%w/v; and V2 = 280ml)

0.125%w/v x V1 = 0.0125%w/v x 280ml

0.0125%w/v x 28ml = 3.5

3.5/0.125%w/v = 28ml

Therefore 28ml of 0.125%w/v

0.125%w/v = 0.125g in 100ml

= 125mg in 100ml

= 35mg in 28ml

(convert from milligrams to micrograms)

= 35,000mcg in 28ml

Learning points

  • The question is asking for the amount and not the concentration
  • The amount of potassium permanganate does not change when going from 1 in 800 to 1 in 8,000. There is still one gram in both. However, the concentration (%w/v) has changed
  • 1 in 800 is 0.125% and not 0.00125%
  • The question asked for the answer in micrograms and the answer was given in milligrams. It is important to always look at the answer grid and the units for the question before answering.
Question two: hydrocortisone dilution

Mrs B, 57, weighs 74 kilograms and has been prescribed 500g of a 1 in 4 dilution of hydrocortisone butyrate 0.1% ointment. The consultant dermatologist wants the pharmacy to prepare the ointment using emulsifying ointment as the diluent.

How many grams of emulsifying ointment are needed to prepare the requested ointment?

Here is the pre-reg’s working out:

1 in 4 = 5 parts

500g/5 = 100g per part

So, one part emulsifying ointment and four parts hydrocortisone cream

The answer is 100g emulsifying ointment

This answer is incorrect. How would you work out the answer?

Click here for the correct working out and answer

Here is one method to work out the answer to this calculation:

1 in 4 = 4 parts

500g/4 = 125g per part

Therefore one part hydrocortisone ointment and three parts emulsifying ointment

3 x 125g = 375g

The answer is 375g emulsifying ointment

Learning points

  • 1 in 4 = 4 parts and not 5 parts
  • It is important not to get the number of each component confused
  • The strength of hydrocortisone butyrate is not important in this calculation, nor is the weight of the patient. Often a question will be worded with information that is not directly relevant to the calculation itself.

Make sure you understand the rounding instructions for GPhC calculations. These can be found on the front of the answer grid. There is also a copy available in the Pre-registration Manual on the GPhC website.

It is very important to have answered the 12 example calculations and understand how to get to the correct answer. Rounding should always occur at the end of the calculation, unless you are working out a dose. Please also look at the learning points from previous sittings for more examples.


Babir Malik is a teacher practitioner at Bradford University and the Green Light Campus masterclass lead

Mr Malik does not set or write questions for the GPhC registration assessment and does not imply that these calculations will come up in the exam. However, both questions relate to the GPhC framework. The author adds that there may be other ways to work out these questions, but it does not matter how you work out a question as long as you get to the correct answer.

Check out C+D’s top tips to prepare for June's GPhC pre-reg exam

You can listen to the podcast in full below. Alternatively, subscribe to all of C+D's podcasts on iTunes by clicking here or by searching 'Chemist+Druggist podcast' on your preferred android podcast app.

GPHC revalidation with chemist and druggist for pharmacy professionals

Where you able to answer the above calculations?

Kamran Sarwar, Community pharmacist

YOUR first worked example answer is wrong. The second worked example is correct for the KMn04 question. Answer is 35000mcg in 28ml NOT 280ml as in the first case.

Roy Sinclair, Community pharmacist

The questions do touch on some of the manupulations that form part of the undergraduate degree but are redundant in practice. When I was training, I can rememeber being asked to find / collect some Dihydrogen Oxide and to beware of spilling it as it was dangerous. I can also remember pre-regs more recently being asked to visit the Triage Nurse in A&E and ask them to visit Pharmacy as they were exceeding 3 patients per bed. Simple Maths now has a very imited place in pharmacy education and practice but remains essential. Questions do need to be up-to-date and relevant to actual practice. How many can comment on the actual cost of a standard commercial IV bag of Sodium Chloride or the cost of scanning a pack under the FMD directives or delivering a medicine to a patients home ? Many Hospital Pharmacists will rarely ever have to consider costs and many Community Pharmacists will only consider them when forced to or when too late. While We continue to concentrate on diluting creams and ointments, the real world mathematical forces at play will continue to dilute our presence and availability but in reality be steadily increasing the burden for everyone else. 

Bob Dunkley, Locum pharmacist

I fear Mr Malik and the GPhC should be updating their questions. Such dilutions were commonplace when I qualified nearly 50 years ago, and even 20 years ago I was making ointments by dilution in community. However, the need to keep batch numbers, product licences etc for things I made extemporaneoulsy  made me rapidly change to a specials manufacturer. Since then, I have never wielded a spatula in anger, as, I suspect, have many of my colleagues in community pharmacy. Even the knowledge of such calculations is redundant. The trouble is, what examples could replace the above?

Female Tech, Pharmacy technician

I'm not even a pharmacist and I could answer them.

NISHI Patel, Community pharmacist

When in community practice do we ever have to make up such products? Everything is sent out for specials. Yes 20 years ago, I even made extemp creams in the dispensary, but not now. Surely the questions should be updated to be relevant to today's practice ?

geoffrey gardener, Community pharmacist

Think we got asked harder questions in our 11 plus. If students are unable to answer such questions, how have they managed to complete a four year degree course?

Dave Downham, Manager

RTFQ - Read The Question.

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