Answer: The new scale factor needs to be 10/3
Explanation:
Consider a line segment that is 10 units long. Apply the scale factor 3/10 to it, and now it's 3 units long because (3/10)*10 = 3.
To get it back to 10 units, we multiply by the scale factor 10/3 which is the reciprocal of the original scale factor. Note that (3/10)*(10/3) = 1. Those two scale factors multiply to 1 to indicate no change has been made overall.
