Explanation:
the €300 as current price is 125% relative to the original price. in other words : 1.25 times the original price.
to get the amount of the increase we need to get the original price first and then subtract the original price from the current price.
to get the original price we need now to divide the actual price by 1.25 to undo the increase.
the original price
OP = 300/1.25 = €240
the price increase (of 25%) was then
300 - 240 = €60