Pond coverage when starting with eight lily pads

A single lily pad placed in an empty pond doubles its area every day, and it covers the whole pond after exactly $10$ days. If instead the pond starts with $8$ such lily pads (assume they never overlap), how many days does it take to cover the pond?

7 days. Eight pads have the same total area that one pad reaches after $3$ doublings ($2^3=8$). So starting with eight is like being $3$ days into the original process, leaving $10-3=7$ days.