What is the scale factor of this dilation?
[not drawn to scale]

How many times bigger is the right triangle than the left triangle?
A side on the right triangle is 15. The same side on the left triangle is 10. 15/10=1.5.
No matter what side you choose on the right triangle and divide it by the corresponding side of the left triangle, you get the same 1.5.
So, the right triangle is 1.5 times larger than the left triangle. This is the scale factor.
What is 1.5 in fraction form?
The answer is left as an exercise for the reader.