Suppose I buy x, then each one is 12/x cents.

Later, I bought a total of (x+2) eggs, each egg 12/(x+2).

Then12/x-12/(x+2) =1.

The value of x can be calculated as 4.

So I bought six.

Special solution equation:

First pass: available

12x+24- 12x=x squared +2x

Transfer items, formulas, available:

(x+6)(x-4)=0

x=4