Difference between revisions of "2021 JMPSC Sprint Problems/Problem 15"

(Solution 2)
Line 12: Line 12:
  
 
~Mathdreams
 
~Mathdreams
 +
 +
 +
 +
==See also==
 +
#[[2021 JMPSC Sprint Problems|Other 2021 JMPSC Sprint Problems]]
 +
#[[2021 JMPSC Sprint Answer Key|2021 JMPSC Sprint Answer Key]]
 +
#[[JMPSC Problems and Solutions|All JMPSC Problems and Solutions]]
 +
{{JMPSC Notice}}

Revision as of 17:15, 11 July 2021

Problem

Find the last two digits of $10^{10}-5^{10}.$

Solution

Note that $10^{10}\equiv0\pmod{100}$ and $5^{10}\equiv25\pmod{100}$.

$0-25=-25$. $-25\equiv\boxed{75}\pmod{100}$

Solution 2

By multiplying out several powers of $5$, we can observe that the last $2$ digits are always $25$ (with the exception of $5^n$ where $n \le 1$). Also, $10^10$ ends with several zeros, so the answer is $100...00 - 25 = 99...99 - 24 = 999...75$.

~Mathdreams


See also

  1. Other 2021 JMPSC Sprint Problems
  2. 2021 JMPSC Sprint Answer Key
  3. All JMPSC Problems and Solutions

The problems on this page are copyrighted by the Junior Mathematicians' Problem Solving Competition. JMPSC.png