An airline allows each passenger 30 kilograms of luggage. Carl has to lower the weight of his suitcase by 25% to stay within the limit. How much did the suitcase originally weigh?

If he had to lower it by 25%, it currently weight 75% of its original weight so if we let w=original weight:

w (75/100) = 30 multiply both sides by 100

75w=3000 divide both sides by 75

w=40

So the original weight of the luggage was 40 kilograms.

30 kg is (100-25) = 75% of the original weight

so original weight = 30 / 0.75 = 40kg