The amount of algae covering the smith's backyard pond doubled every day until it was completely covered in algae on day 30 of the month. on what day of that month was 75% of the pond algae-free?

To solve this problem, work backwards.

On day 30, it is 100% algae.

On day 29, it is 50% algae.

On day 28, it is 25% algae(or 75% algae free).  

It was on Day 28.

answer: 42

use a calculator to find that 845/73 = 11.575

the whole part of the answer is 11. multiply this by 73 to get 11*73 = 803.

subtract that result from 845 to get 845 - 803 = 42