An airplane allows each passenger 36 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 weight?

Let the original suitcase weight be x.  This x will be reduced by multiplying it by (1.00-0.25), or 0.75.  0.75x = 36 kg.  Solve for x.

Dividing both sides of this last equation by 0.75, we get  x = 48.

Carl's suitcase originally weighed 48 lb.  25% of that is 12.  x less 12 is 36 lb, which is acceptable to the airline.