On which day would it cover 1/81 of the pond?

Question

A lily in a pond trebles its size every 3 days. It covered the pond completely on a Friday. On which day would it cover 1/81 of the pond?

 

Answer 1

Work backwards...

 

If it triples its size every day, then the day BEFORE it covered the pond completely it would have covered 1/3 of the pond.  The day before that, 1/3 of 1/3, or 1/9 of the pond.  The day before that (3 days ago), 1/3 of 1/9, or 1/27 of the pond. 

 

And finally, the day before that, 4 days ago, it would have covered 1/3 of 1/27 of the pond, or 1/81.

 

Four days before Friday is Monday.