What is the greatest common multiple of 25 and 75?

Answer: Infinity or there is none.

One common multiple of 25 and 75 is 1875. If you double it, which is 3750, you have a larger common multiple. If you double it again, which is 7500, you have an even larger common multiple. You can continue doubling it forever, so there is no greatest common multiple, or it can be said to be infinite.