After a long hiatus due to my writing of the summer plot, RotD returns...
Two strangers from different parts of the country are building condominiums. By chance, they used the same architect and the same contracting company to build their condos. The two strangers notice that they are missing the same part(s), so, by chance, they call the same hardware store. They are quoted these prices:
One will cost them $2
Two will cost them $2
Thirteen will cost them $4
One Thousand Six Hundred and Twenty Five will cost them $8.
What did they neglect to purchase?
The numbers to put on the house, each number costs them $2, so 1625= 4 numbers, $8.