This is a favorite type of homeschool math challenging puzzle — The Snail question.
Question: An aquarium is 47 cm deep. A snail starts at the bottom of the aquarium. Each day, during the daytime the snail climbs up 8 cm, and during the nighttime the snail slides down 3 cm. How many days will it take for the snail to climb out of the aquarium?
This is part of a series on Homeschool Math Challenging Puzzles, suitable for Grades 2-4. (Of course, students of other grades are also welcome to try them out.) The questions are suitable for:
- Homeschooling for gifted kids
- Preparation for GEP (Gifted Education Programme) screening and selection tests
- Preparation for Math Olympiad
- Puzzles for kids interested in math but find school work too easy.
A tempting answer would be 10 days. This is a trick! Those who get the answer “10 days” reason like this: each day the snail moves a net distance of 8-3=5 cm.
Hence, 47/5=9 R 2.
At the end of the 9th day, the snail moved 45 cm. Thus, rounding up will give 10 days as the answer. However, there is a tricky part to the question!
The correct answer is 9 days.
We can do a list:
Day 1 — 5 cm
Day 2 — 10 cm
Day 3 — 15 cm
Day 8 — 40 cm
Day 9 day time — 40 +8 = 48cm > 47 cm !!!
The snail is already out of the aquarium on Day 9!