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What types of questions can appear with percentage calculations?

Mainly relating to A-Level Biology:
With biology, I always get percentage calculation questions incorrect because I don't know when to use the percentage increase or decrease formula or when to just find what percentage X Value is of Y Value. For example (question 1bi): https://pmt.physicsandmathstutor.com/download/Biology/A-level/Topic-Qs/AQA-Old/Unit-1/Set-2/Lungs%20&%20Diseases.pdf using the initial-final/final x 100 formula u get 76 - 36 / 36 (x100) and get 111%
Then u try 36-76/76 and get 52.6%. but you are meant to go with Dividing 36/76 = 0.47 then x100 = 47%. Please can someone clarify what type of percentage questions can come up and how I shouldl deal with them and whart they are asking? Specifically AQA A-Level Biology if possible. Also with graphs which show results like ' U get X Value if u increase temperature' and 'U get Y value if u do not increase temperature', which value do I use for my 'percentage increase' you don't know which percentage increase it is from, like increase from X to Y, or increase from Y to X??? Thank u so much if u can help
Reply 1
For the question you gave, what's on the y-axis is already a percentage, so you just have to subtract 35 from 75, giving a difference of 40%.

I find that thinking of percentage calculations like substituting values into a formula overcomplicates things, so I find it more intuitive to think about these calculations logically. When finding any percentage calculation, I begin by dividing the new or changed value, y by the initial value, x. This gives me y as a proportion of x, the same as y as a percentage of x if I were to multiply by 100%. If I want the percentage increase, I take away 1 and multiply by 100% (or multiply by 100% then take away 100% it doesn't matter), or if I want the percentage decrease, I take what I calculated from 1 and multiply by 100%.

For what you said about calculations from "X" and "Y", Y is the "control" value, it's the original value at the standard temperature whereas X is what you get when the temperature is increased / changed, so you calculate the percentage increase from Y to X.
Reply 2
Original post by 0gg
For the question you gave, what's on the y-axis is already a percentage, so you just have to subtract 35 from 75, giving a difference of 40%.
I find that thinking of percentage calculations like substituting values into a formula overcomplicates things, so I find it more intuitive to think about these calculations logically. When finding any percentage calculation, I begin by dividing the new or changed value, y by the initial value, x. This gives me y as a proportion of x, the same as y as a percentage of x if I were to multiply by 100%. If I want the percentage increase, I take away 1 and multiply by 100% (or multiply by 100% then take away 100% it doesn't matter), or if I want the percentage decrease, I take what I calculated from 1 and multiply by 100%.
For what you said about calculations from "X" and "Y", Y is the "control" value, it's the original value at the standard temperature whereas X is what you get when the temperature is increased / changed, so you calculate the percentage increase from Y to X.

Can u explain why u take calculated value away from 1 for decrease and 1 away from answer for increase? If I'm not wrong is the logic, if Y is 112% of X, then Y is of 12% percentage increase from X?
Reply 3
Original post by rishoo1
Can u explain why u take calculated value away from 1 for decrease and 1 away from answer for increase? If I'm not wrong is the logic, if Y is 112% of X, then Y is of 12% percentage increase from X?

Let's say sales at a company decrease from 100 a week to 70 a week. From those numbers, it's easy to see that there's a 30% decrease in sales. Doing 70 / 100 = 0.7 then 1 - 0.7 is the same as doing 100 - 70 = 30 (the difference in sales between the two weeks) then 30 / 100 = 0.3 to get the percentage: 0.3 x 100% = 30%.

What you said about the 12% increase is exactly right (:

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