A bacterial cell divides every minute. It was found that it filled the petri-plate in half-an-hour. In how much time was the plate filled one-fourth ?

A bacterial cell divides every minute. It was found that it filled the petri-plate in half an-hour. As next minute that is at 29 min the plate will be $2 \times 1/4=1/2$ and next minute that  is 30 min the plate will be full whole ie $2\times 1/2=1$.