Our universe is "9- billion years across"? "So it would take 25,000 years to get there if you traveled at the speed of light."

I would like the mathematics to this explained. To the observer on earth, it would take 9 billion years for light to travel 9 billion light years. To the traveller moving at 99% the speed of light you would need to use a factor of (I think) around 15% so, to them, it would be around 1.3 billion years.

My knowledge stops there as I don't know what the factor is for travelling at the actual speed of light! What is it? Simply that is :-)