The perimeter of a rectangle changes from 2.5 to 10 after dilation what was the scale factor of the dilation

The scale factor is 4 (10/2.5).

Each side has increased by a factor of 4. Think of it logically, if the original rectangle had sides of 1,1,1/4,1/4, the perimeter would add to 2.5. In order to get the sides to add up to 10, each of the sides would need to be multiplied by 4. Then our sides would measure 4,4,1,1 with a perimeter of 10.