其它
A kind of water plant grows so fast that it doubles every day. If one plant is put into the pond on the first day, two plants will grow on the second day, and on the twenty-sixth days, they will just fill the pond. If eight water plants are put into the pond on the first day, how many days it will take to fill the pond? 
Overseas Competition->Knowledge Point->Number Theory Modules
It takes three days for one plant to develop to eight plants. 