FILE No.103Unsolved
Algae is growing in a pond. Every day it triples its area. On day 20, the whole pond was covered.
On what day was one third of the pond covered?
If you get stuck, open the hints one at a time, in order.
What fraction was it the day before?
Think of the opposite of tripling.
What happens when one third of a pond triples?
On what day was one third of the pond covered?
Choose the answer closest to the truth.
Day 19
The answer is "Day 19."
Because the area triples each day, the day before the pond is fully covered it was one third covered. If the whole pond is covered on day 20, then day 19 is the day with one third.
On day 7 or day 10, tripling each day would never reach the full pond on day 20. Day 18 is one ninth.
Because you divide instead: "one third of 20 is about day 7."
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