Each person engaged in the business of selling adding machines, calculating machines, comptometers, billing machines, bookkeeping machines, cash registers, typewriters, or similar machines shall pay the following annual privilege tax: In counties of over 100,000 inhabitants, $100; in counties of over 60,000 inhabitants and not over 100,000 inhabitants, $60; in counties of over 40,000 inhabitants and not over 60,000 inhabitants, $40; in counties of 40,000 inhabitants or less, $25.