Final answer:
The percent increase in the membership cost at 24 Hour Fitness is 25%.
Step-by-step explanation:
To find the percent increase, we need to calculate the difference between the old cost and the new cost, then divide that by the old cost and multiply by 100. In this case, the difference is $100 - $80 = $20, so the percent increase is ($20/$80) * 100 = 25%. Therefore, the correct answer is a) 25%.