Please can anyone help with basic Maths (percentages)

[Grapetree]

I struggle a lot with maths, so hopefully someone else will find this straight forward

Q) if the average amount spent by the 15-25 age group decreases from August to September by the same % increase from June to July what would be the average amount spent by that group in September?

(Picture of a graph for Q included)
I think have managed to work out the first part of the question - the difference in spending from June to July is 50% (the figures from June were £60, July £90)
^ I calculated it by 30/60 = 0.5 x 100

However now I am very lost on how to calculate the second part I.e. how would I calculate a 50% decrease of £180
(£180 is the spending for August I was given on graph btw)
At first I thought it was 180 x 0.5 so £90 A’s the answer but that doesn’t seem correct to me

£90 is correct. Why doesn't it seem right?

So they spent:

June - £60
July - £90
August - £180
September - £90

Is the amount on the graph the same for July and September? If it's a proper graph, it should be.

Are you confused because an increase of 50% from £60 to £90 is £30 but a decrease of 50% from £180 is a decrease of £90?

That's a common cause of confusion, but the amounts are different because they are relative to different amounts, ie 50% of £60 vs 50% of £180. Half of £180 is always going to be more than half of £60 .

Ahh yes, you seemed to have nailed exactly what I was finding confusing! I couldn’t think on how to word why it seemed wrong to me 😅

Thank you so much for your help!