Solution to March’s Puzzle: Too Clever by Half
A chicken and a half lay an egg and a half in a day and a half. How
many eggs would one chicken lay in three days? A builder and a half build
a house and a half in a year and a half using a tool and a half. How many
houses would one builder build in nine years? Can you generalize your
calculation to solve both equations?
The intuitive yet incorrect answer to the chicken and eggs puzzle is three
eggs. Our brain plays a trick on us and makes us think that if a chicken and
a half lay an egg and a half in a day and a half, one chicken lays one egg
in one day. But if you express the relationship between chickens, days, and
eggs mathematically, you get the equation
3/2 chickens × 3/2 days = 3/2 eggs
Reducing the number of chickens from 3/2 to 1 is achieved by dividing the
original number by 3/2. For the equation to be true, you also need to divide
the number of eggs (3/2) by 3/2, giving you the equation
1 chicken × 3/2 days = 1 egg
Reducing the number of days from 3/2 to 1 has a similar effect on the
number of eggs; namely, you need to divide 1 (egg) by 3/2, giving you the
equation
1 chicken × 1 day = 2/3 egg
If you increase the number of days from 1 to 3, the effect on the number of
eggs is a factor of 3 as well:
1 chicken × 3 days = 2 eggs
In a very similar manner you can express the relationships between
builders, houses, years, and tools with the following equation:
3/2 builders × 3/2 years × 3/2 tools = 3/2 house
To reduce the number of builders, years, and tools to one each, you need to
divide the number of houses by 3/2 three times; in other words, by (3/2)^3:
1 builder × 1 year × 1 tool = 3/2 ÷ 3/2 ÷ 3/2 ÷ 3/2 houses
Giving you the equation
1 builder × 1 year × 1 tool = 4/9 house
Thus, one builder with one tool will build four houses in nine years.
To generalization the equation, you need to divide the right side of the
equation by 3/2 n times for n elements in the left side of the equation. Or,
if you want to express the calculation as a multiplication instead of division,
multiply by (2/3)^n. For example, in our last equation:
3/2 builders × 3/2 years × 3/2 tools = 3/2 houses
The left side of the equation contains three elements; therefore
1 builder × 1 year × 1 tool = 3/2 × (2/3)^3 houses
which is equal to
1 builder × 1 year × 1 tool = 4/9 house
April’s Puzzle: Suicidal Mosquito
Two trains drive toward each other on the same rail. Both trains drive at a
speed of 100MPH. When the trains are 100 miles apart, a mosquito starts
flying back and forth from the front of one train to the other at a speed of
200MPH. What total distance will the mosquito cover before the two
trains crash?