25 pupils solve a problem in 4 hours. For how many same problem will 30 pupils solve?

4 hours because from the fact that the number of pupils of nothing not to a menyaetsyatozha in 4 hours increases or on the contrary. Even if the added 5 pupils will solve a problem in 5 min., then other 25 pupils will not grow wiser - and will still think over the unique task 4 hours
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