2020 AMC 8 Problem 22
Below is the video solution and professionally curated solution for Problem 22 of the 2020 AMC 8, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2020 AMC 8 solutions, or check the answer key.
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Difficulty rating: 1670
22.
When a positive integer is fed into a machine, the output is a number calculated according to the rule shown below.
For example, starting with an input of the machine will output Then if the output is repeatedly inserted into the machine five more times, the final output is When the same -step process is applied to a different starting value of the final output is What is the sum of all such integers
Video solution:
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Written solution:
To see which numbers we can make by inverting it, let's make an inverting machine.
This would take and yield either if is even (which it always is) or if is an odd integer. Note that is an integer only if Also, if is even, then is odd. That would mean would be odd. Therefore, our inverter machine yields and also if and is even.
Now, we must see what the inverting machine can yield after moves:
We can only get
From we can only get
From we can get and
From we can get only ; from we can only get
From we can get only ; from we can get and
From we can get and ; we can get ; from we can get
Move can yield and and their sum is
Thus, the correct answer is E.
Problem 22 in Other Years
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