Here is question 18 from the AQA 3F June 2017 paper:
The first thing to notice is this question is worth a whopping 5 marks and there is that tell-tale sentence ‘You must show your working.‘, so it is going to require some thought and thorough working out. Before committing anything to paper have a think about what the question is asking, how to go about answering it and how best to lay out your answer.
Lets consider these in more detail:
What is the question asking?
You need to know which brand is better value, which means you need to work out which battery will power the toy for some amount of time, say 1 hour, for less money. So there is a price to consider and also time. If you work out the cost of powering the toy with one battery for one hour for each brand, you will have 2 directly comparable numbers.
The maths required
You must compare like with like, which is the opposite of what you have at the moment as the number of batteries in each pack are different and the amount of time each battery lasts for is different.
Firstly, calculate the price per battery for each brand, this is the cost of the pack divided by the number of batteries in the pack. Next calculate the cost of running the toy with one battery for 1 hour, this is the cost of 1 battery divided by the time the battery will last. Now you can compare these two numbers and see which is the lowest. The lower number is the best value brand.
Laying out the answer
Its very tempting to write the maths down without any words, as shown in this worked example:
Try NOT to do this. Although this solution gets the correct answer and uses the correct numbers, it may not get all 5 marks. There is nothing to indicate what is being worked out: which brand is which? what quantities are being worked out? and what are the units? Also, the maths is wrong as 3.60/8 is 0.45 (which is the price of one battery for brand A in pounds). This answer has then immediately been divided by 5 to get the price per hour as £0.09, which is correct, BUT 3.6/8 IS NOT EQUAL to 0.45/5.
Here is a much better solution:
The answer is well presented and is clear to follow as there are clear headings to denote which brand the workings are for. The words describe what is being calculated and there are units. There is a sentence at the end which gives the reason why brand B batteries are better value.