Pick a number. Say 25. Now break it up into as many pieces as you want. 10, 10, and 5, maybe. Or 2 and 23. Twenty-five ones would work. Now multiply all those pieces together.
What's the biggest product you can make?
What's your strategy?
Will it always work?
Does it work for fractions?
Is there another set of numbers you could use that isn't explicitly against the stated rules?