Difference between revisions of "2021 AMC 12A Problems/Problem 5"

Revision as of 15:52, 11 February 2021

Problem

When a student multiplied the number $66$ by the repeating decimal $\underline{1}.\underline{a}\underline{b}\underline{a}\underline{b}...=\underline{1}.\overline{\underline{a}\underline{b}},$ where $a$ and $b$ are digits, he did not notice the notation and just multiplied $66$ times $\underline{1}.\underline{a}\underline{b}$ . Later he found that his answer is $0.5$ less than the correct answer. What is the $2$ -digit number $\underline{a}\underline{b}?$

$\textbf{(A) }15 \qquad \textbf{(B) }30 \qquad \textbf{(C) }45 \qquad \textbf{(D) }60 \qquad \textbf{(E) }75$

Solution

It is known that $0.\overline{ab}=\frac{ab}{99}$ and $0.ab=\frac{ab}{100}$ . Let $\overline {ab} = x$ . We have that $66(1+\frac{x}{100})+0.5=66(1+\frac{x}{99})$ . Solving gives that $100x-75=99x$ so $x=\boxed{(E) 75}$ . ~aop2014

Video Solution by Punxsutawney Phil

https://youtube.com/watch?v=MUHja8TpKGw&t=359s

Video Solution by Hawk Math

https://www.youtube.com/watch?v=P5al76DxyHY

2021 AMC 12A (Problems • Answer Key • Resources)
Preceded by Problem 4	Followed by Problem 6
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
All AMC 12 Problems and Solutions

These problems are copyrighted © by the Mathematical Association of America, as part of the American Mathematics Competitions.

@@ Line 9: / Line 9: @@
 ==Video Solution by Punxsutawney Phil==
 https://youtube.com/watch?v=MUHja8TpKGw&t=359s
+==Video Solution by Hawk Math==
+https://www.youtube.com/watch?v=P5al76DxyHY
 ==See also==
 {{AMC12 box|year=2021|ab=A|num-b=4|num-a=6}}
 {{MAA Notice}}

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