Take it from me and believe what I am saying and go get your animal seen by a doctor. One of the most common ailments for cats as they grow older is renal or kidney disease. If you have an older cat, one of the first symptoms of renal failure is drinking a lot of water
First of all, to the previous answer...You obviously know nothing about cats and I hope you don't have one! Now to answer the question...I have been doing a lot of research online trying to figure out the same problem in my cat. She too is 15 and is drinking excessive amount of water. I had blood tests done and her kidneys and glucose levels are normal. The excessive drinking has been a symptom of diabetes, hyperthyroidism and kidney failure. The latter is incurable but the former 2 can be treated easily. Take your cat to the vet to be sure so you can alleviate the problem soon.
I'm no mathematician but I would have to say your cat is thirsty.