There are 32 places for the teeth in the mouth. For each place, we have two choices either there is a tooth or there is no tooth.
Choices of 1 teeth =2
Again Choices of 2 teeth =2×2=22
Choices of 3 teeth =2×2×2=23
Similarly Choices of 32 teeth =232
which also includes person without teeth
But given that no person is without a tooth
Hence, the maximum population of the city =232−1.
Now k=nm−1
So m=32 and n=2
Hence, mn=16