Compute successive differences of the terms.
If they are all the same, the sequence is arithmetic and the common difference is the difference you have found.
If successive pairs of differences have the same ratio, the sequence is geometric and the common ratio is the ratio you have determined.
Example of arithmetic sequence:
1, 3, 5, 7
Successive differences are 3-1 = 2, 5-3 = 2, 7-5 = 2. All the differences are 2, which is the common difference of the sequence.
Example of geometric sequence:
1, -3, 9, -27
Successive differences are -3-1 = -4, 9-(-3) = 12, -27-9 = -36. These are not the same, so the sequence is not arithmetic. Ratios of successive pairs of differences are 12/-4 = -3, -36/12 = -3. These are the same, so the sequence is geometric with common ratio -3.
Answer:
50% profit
Step-by-step explanation:
Cost price = £ 5000
Selling price = £ 7500
Profit = Selling price - cost price
= 7500 - 5000
= £ 2500
Profit percentage = 
= 50 %
You have not given us any of the steps that Ricardo took to simplify the
expression, and you also haven't given us the list of choices that includes
the description of his mistake, so you're batting O for two so far.
Other than those minor details, the question is intriguing, and it certainly
draws me in.
If Ricardo made a mistake in simplifying that expression, I'm going to say that
it was most likely in the process of removing the parentheses in the middle.
Now you understand that this is all guess-work, because of all the stuff that you
left out when you copied the question, but I think he probably forgot that the 3x
operates on everything inside the parentheses.
He probably wrote that 3x (x-3) is
either 3x² - 3
or x - 9x .
In reality, when properly simplified,
3x (x - 3) = 3x² - 9x .
