You save 15 dollars.
23
If someone loans you 50 dollars, which you then spend, you will owe them 50 dollars which could be represented as you having -50 dollars. If you are at an elevation of 0 feet above sea level, then dive below the water 50 feet, your elevation would be -50 feet.
50% of 52 dollars = 52*50/100 dollars = 26 dollars.
25% off of 50 dollars = 75% of 50 dollars = 50*75/100 = 37.50 dollars