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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Concepts:work backwardstree diagram

Difficulty rating: 1670

22.

When a positive integer NN is fed into a machine, the output is a number calculated according to the rule shown below.

For example, starting with an input of N=7,N=7, the machine will output 37+1=22.3 \cdot 7 + 1 = 22. Then if the output is repeatedly inserted into the machine five more times, the final output is 26.26. 7221134175226 \begin{align*} &7 \to 22 \to 11 \to 34 \\ &\to 17 \to 52 \to 26 \end{align*} When the same 66-step process is applied to a different starting value of N,N, the final output is 1.1. What is the sum of all such integers N?N? N00000000001 \begin{align*} &N \to \underline{\phantom{00}} \to \underline{\phantom{00}} \to \underline{\phantom{00}}\\ &\to \underline{\phantom{00}} \to \underline{\phantom{00}} \to 1 \end{align*}

7373

7474

7575

8282

8383

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 NN and yield either 2N2N if 2N2N is even (which it always is) or N13\frac{N-1}{3} if N13\frac{N-1}{3} is an odd integer. Note that N13\frac{N-1}{3} is an integer only if N1mod3.N \equiv 1 \mod 3. Also, if NN is even, then N1N-1 is odd. That would mean N13\frac{N-1}{3} would be odd. Therefore, our inverter machine yields 2N2N and also N13\frac{N-1}{3} if N1mod3N \equiv 1 \mod 3 and NN is even.

Now, we must see what the inverting machine can yield after 66 moves:

1)1) We can only get 2.2.

2)2) From 2,2, we can only get 4.4.

3)3) From 4,4, we can get 11 and 8.8.

4)4) From 1,1, we can get only 22; from 8,8, we can only get 16.16.

5)5) From 2,2, we can get only 44; from 16,16, we can get 55 and 32.32.

6)6) From 4,4, we can get 11 and 8,8,; 5,5, we can get 1010; from 3232 we can get 64.64.

Move 66 can yield 1,8,10,1, 8, 10, and 64,64, and their sum is 83.83.

Thus, the correct answer is E.

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