2021 AMC 10A Spring Problem 3

Below is the video solution and professionally curated solution for Problem 3 of the 2021 AMC 10A Spring, from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 2021 AMC 10A Spring solutions, or check the answer key.

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Concepts:place valuelinear equation

Difficulty rating: 900

3.

The sum of two natural numbers is 17,402.17,402. One of the two numbers is divisible by 10.10. If the units digit of that number is erased, the other number is obtained. What is the difference of these two numbers?

10,27210,272

11,70011,700

13,36213,362

14,23814,238

15,42615,426

Video solution:
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Written solution:

Let xx and yy be the two numbers. WLOG, let xx be divisible by 10.10. Then the units digit of xx is 0.0.

If we erase the units digit, then we are essentially dividing xx by 10.10. The problem statement also gives us that x10=y.\dfrac{x}{10} = y.

Therefore, x10+x=17,40211x10=17,402x=15,820. \begin{align*} \dfrac{x}{10} + x &= 17,402 \\ \dfrac{11x}{10} &= 17,402 \\ x &= 15,820. \end{align*} Then xx10=14,238. x - \dfrac{x}{10} = 14,238.

Thus, D is the correct answer.

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