I'm not sure I totally understood the problem but from what I could catch:
If the output was at year n, then the output will be at the following year since we have a decrease by 25%. The question is asking for the total future output. What we have is a geometric sequence with a first term of 25,000,000 and a ratio of 0.75. The total sum of of this sequence is given by (since we are summing to infinity). The answer of the book seems to be wrong unless I misunderstood the problem. By the way, is this the value you are getting??