Answer:
The penny size d is given by the literal equation

Where n is the length (in inches) of the nail.
We can deduce the value of n from equation above:

Now we have to find n with various values of d.
1. when d = 3 inches


n = 1.25 inches
2. when d = 6 inches


n = 2 inches
3. when d = 10 inches


n = 3 inches