For the extra cost of feeding one additional person, given the formula Q = 15 + 0.5m, where Q is the total cost in Naira and m is the number of people.

The cost for $m$ people is given by the formula:

$$Q_m = 15 + 0.5m$$

If one additional person is fed, the number of people becomes $m+1$. The new cost, $Q_{m+1}$, is:

$$Q_{m+1} = 15 + 0.5(m+1)$$ $$Q_{m+1} = 15 + 0.5m + 0.5$$

The extra cost is the difference between the cost for $m+1$ people and the cost for $m$ people:

$$\text{Extra Cost} = Q_{m+1} - Q_m$$ $$\text{Extra Cost} = (15 + 0.5m + 0.5) - (15 + 0.5m)$$ $$\text{Extra Cost} = 15 + 0.5m + 0.5 - 15 - 0.5m$$ $$\text{Extra Cost} = 0.5$$

The extra cost of feeding one additional person is $\text{N}0.50$.