ёпт, ну и условие=(

Statement is more than clear. Solution is not that trivial.

I can not understand this string "Help Vova to find such M, which maximizes the probability that exactly m out of n random passers-by speak Cantonese." What does it mean?

This means exactly what is written - for any possible M there is some probability that, when taken n people out of N, exactly m of them will speak Cantonese. You want to estimate M in a way to maximize that probability. This is called maximum likelihood approach in statistics.