The Average Cost of producing 5 units

The Average Cost of producing 5 units of a good is Rs. 6 and that of producing 6 units is Rs. 5. What is the Marginal Cost of producing 6 units?

TC = AC x Q or MC = (6x 5) - (5 x 6) = 30- 30= 0
MC of producing 6 units = 0