2020 AMC 8 Problems/Problem 20
A scientist walking through a forest recorded as integers the heights of trees standing in a row. She observed that each tree was either twice as tall or half as tall as the one to its right. Unfortunately some of her data was lost when rain fell on her notebook. Her notes are shown below, with blanks indicating the missing numbers. Based on her observations, the scientist was able to reconstruct the lost data. What was the average height of the trees, in meters?
Contents
Solution 1
It is not too hard to construct as the heights of the trees from left to right. The average is
.
Solution 2
For the heights of the trees to be integers, we must have the height of Tree 1, Tree 2, Tree 3 as . Now, there are two possible cases.
Case 1: The length of tree 4 is -
So, we have the first 4 tree lengths as
. For the length of Tree 5 to be an integer, we must have the length of Tree
as
. But, when we take the average of these
integers, it results in
, which doesn't satisfy our conditions.
Case 2: The length of tree 4 is -
So, we have the first
tree lengths as
. Now, using quick modular arithmetic, we see that when the length of Tree 5 is
, the average of the heights of the 5 trees is
. This is where our condition is satisfied.
~ATGY
Solution 3
Let the sum of the heights of the trees be denoted by . The average height will then be
. Since the average height has decimal part
, it follows that
must have a remainder of
when divided by
. Thus,
must be
more than a multiple of 5. Since
is an integer, it follows that the heights of Tree 1 and Tree 3 are likely both 22. At this point, our table looks as follows:
If we guess that Tree 4 has a height of 11, we would need to make Tree 5 have a height of 22. In this case would equal
which is not one more than a multiple of
. So we instead guess that Tree 4 has a height of
. Then we only need to try out heights of
and
for Tree 5. Using a height of
for Tree 5 gives us
which is
more than a multiple of
. Thus,
and the average height of the trees is
meters
.
~ junaidmansuri
See also
2020 AMC 8 (Problems • Answer Key • Resources) | ||
Preceded by Problem 19 |
Followed by Problem 21 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
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