Difference between revisions of "2021 AMC 10A Problems/Problem 11"

Revision as of 18:39, 5 May 2021

Problem

For which of the following integers $b$ is the base- $b$ number $2021_b - 221_b$ not divisible by $3$ ?

$\textbf{(A)} ~3 \qquad\textbf{(B)} ~4\qquad\textbf{(C)} ~6\qquad\textbf{(D)} ~7\qquad\textbf{(E)} ~8$

Solution 1

We have $\begin{align*} 2021_b - 221_b &= (2021_b - 21_b) - (221_b - 21_b) \\ &= 2000_b - 200_b \\ &= 2b^3 - 2b^2 \\ &= 2b^2(b-1), \end{align*}$ which is divisible by $3$ unless $b\equiv2\pmod{3}.$ The only choice congruent to $2$ modulo $3$ is $\boxed{\textbf{(E)} ~8}.$

~MRENTHUSIASM

Solution 2 (Easy)

Vertically subtracting $2021_b - 221_b$ we see that the ones place becomes 0, and so does the $b^1$ place. Then, we perform a carry (make sure the carry is in $base b$ !). Let $b-2 = A$ . Then, we have our final number as $1A00_b$

Now, when expanding, we see that this number is simply $b^3 - (b - 2)^2$ .

Now, notice that the final number will only be congruent to $b^3-(b-2)^2\equiv0\pmod{3}$ if either $b\equiv0\pmod{3}$ , or if $b\equiv1\pmod{3}$ (because note that $(b - 2)^2$ would become $\equiv1\pmod{3}$ , and $b^3$ would become $\equiv1\pmod{3}$ as well, and therefore the final expression would become $1-1\equiv0\pmod{3}$ . Therefore, $b$ must be $\equiv2\pmod{3}$ . Among the answers, only 8 is $\equiv\pmod{3}$ , and therefore our answer is $\boxed{\textbf{(E)} ~8}.$

- icecreamrolls8

Video Solution (Simple and Quick)

https://youtu.be/1TZ1uI9z8fU

~ Education, the Study of Everything

Video Solution

https://www.youtube.com/watch?v=XBfRVYx64dA&list=PLexHyfQ8DMuKqltG3cHT7Di4jhVl6L4YJ&index=10

~North America Math Contest Go Go Go

Video Solution 3

https://youtu.be/zYIuBXDhJJA

~savannahsolver

Video Solution by TheBeautyofMath

https://youtu.be/t-EEP2V4nAE

~IceMatrix

@@ Line 5: / Line 5: @@
 ==Solution 1==
-We have <cmath>2021_b - 221_b = 2000_b - 200_b = 2b^3 - 2b^2 = 2b^2(b-1).</cmath> This expression is divisible by <math>3</math> <b><i>unless</i></b> <math>b\equiv2\pmod{3}.</math> The only choice congruent to <math>2</math> modulo <math>3</math> is <math>\boxed{\textbf{(E)} ~8}.</math>
+We have
+<cmath>\begin{align*}
+_b - 221_b &= (2021_b - 21_b) - (221_b - 21_b) \\
+&= 2000_b - 200_b \\
+&= 2b^3 - 2b^2 \\
+&= 2b^2(b-1),
+\end{align*}</cmath>
+which is divisible by <math>3</math> <b><i>unless</i></b> <math>b\equiv2\pmod{3}.</math> The only choice congruent to <math>2</math> modulo <math>3</math> is <math>\boxed{\textbf{(E)} ~8}.</math>
 ~MRENTHUSIASM

2021 AMC 10A (Problems • Answer Key • Resources)
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