If you get 50 questions, that means you get all 12%.
You add that you're current percentage: 73.2% + 12% = 85.20%
However, if you think you'll only get 25, then you'll get:
(25/50) (12%) = 6%
You add this to your current percentage:
73.20% + 6% = 79.20%
Refer to you passing score and see if 79.20% gives you a passing mark.