you have old_spending * 1.25 = new_spending_required_for_old_level
and old_spending_with_return = old_spending / new_spending_required_for_old_level = 1/1.25
the later is your result. (or better to say: 1- 1/1.25)
Maybe follow this line of thought.
Lets say $10 used to buy you 8 litres of petrol.
Now with a 25% increase implies $12.50 will now buy you 8 litres of petrol. You need to know how many litres to buy at the new price such that you are still only spending $10.
So 8 litres at $12.50 is $1.56 a litre.
So you need to know $1.56 by X (litres) = $10