Mrs. morton has a special reward system for her class. when all her students behave well, she rewards them by putting 3 marbles into a marble jar. when the jar has 100 marbles or more, the class has a party. right now, the jar contains 24marbels

Not sure what you're asking about this but you would need 76 more marbles to get 100. That means that you would need to be rewarded 26 more times. It would also mean that they've been reward 8 times already. 

If you had to do it this way you would say 24/100 reduced to 6/25 that's 24% of the way complete. That would leave 76% incomplete.  

Also, no matter what you'd have to get over 100 marbles to get rewarded because if she gives 3 at a time, then it doesn't come out even. The minimum you could get is 102 marbles.