Which of the following is both a perfect square and a perfect cube? 64
16
81
1,000

I know the answer is 64, but how? Can someone please explain me to me how to work this out?

Ascientist has discovered an organism that produces five offspring exactly one hour after its own birth, and then goes on to live for one week without producing any additional offspring. each replicated organism also replicates at the same rate. at hour one, there is one organism. at hour two, there are five more organisms. how many total organisms are there at hour seven? 2,801 19,531 19,607 97.655