



 
Silje, First, we have to note that the question is very unrealistic; the cylinder would contain $2^{120}$ peas (about $10^{36}$ peas, a thousand times the mass of the solar system!) Makes the chessboard story (one grain of wheat on the first square, etc...) look pretty tame. Now, how to solve it. Consider some smaller numbers: if your cylinder is full after the third minute (1+2+4 peas) about when is it halffull? If it's full after the fourth minute, about when is it halffull? If it's full after the fifth minute, about when is it halffull? You should be able to answer the question yourself now!  


Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences. 