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.