9.1. If Petya performs a certain task in such a way that each subsequent day he does twice as much as in all the previous days combined, then the entire task will take him 12 days. How many days will it take for Petya and Kolya to complete this task if Kolya works in the same way as Petya?

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9.1. It is clear that in 11 days, Petya will complete exactly half of the entire work, since on the twelfth day, he must complete as much work as he did in the previous 11 days combined, after which the entire work will be completed. If Kolya works together with Petya, then Kolya will also complete half of the entire work in 11 days. Thus, both boys together will complete the entire work in 11 days.