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FoolForMath
 2 years ago
Another very easy problem,
Apoor, a four year old child, had to write \(1\) to \( n\) numbers as a punishment. If he had made a mistake while writing any number, say \(r\), then he had to write that number again \((r1)\) times, which he always wrote correctly. If he had made mistakes while writing all except one number and the total numbers he wrote is \(111\), then find the number that he wrote correctly.
FoolForMath
 2 years ago
Another very easy problem, Apoor, a four year old child, had to write \(1\) to \( n\) numbers as a punishment. If he had made a mistake while writing any number, say \(r\), then he had to write that number again \((r1)\) times, which he always wrote correctly. If he had made mistakes while writing all except one number and the total numbers he wrote is \(111\), then find the number that he wrote correctly.

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FoolForMath
 2 years ago
Best ResponseYou've already chosen the best response.0I have a feeling that @KingGeorge will solve this one under 2 minutes.

KingGeorge
 2 years ago
Best ResponseYou've already chosen the best response.4I'm gonna say that he wrote 10 correctly.

KingGeorge
 2 years ago
Best ResponseYou've already chosen the best response.4Since he wrote very number except one incorrectly, we can say he wrote \[1+2+3+4+...+n(r1)=111\]numbers total. Where \(r\) is the number he wrote correctly. If we look at the triangular numbers at http://oeis.org/A000217 we see that \(T_{15}=120\) is the smallest triangular number greater than 111. The next is \(T_{16}=136\). If it were \(T_{16}\) he would have had to have written 25 down correctly, but he can't have done that since \(25>16\). Thus, he wrote the numbers up to 15, and wrote \(120(r1)=111\) numbers total. Solving this, we get \(r=10\).

KingGeorge
 2 years ago
Best ResponseYou've already chosen the best response.4Typo. If it were \(T_{16}\) he would have had to written 26 down correctly. Not 25.

KingGeorge
 2 years ago
Best ResponseYou've already chosen the best response.4I think it's worth noting that this problem has a solution for every possible number of "total numbers" he wrote.
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