Silly one

Algebra Level 2

The average of first 81 natural number is

40 42 40.5 41

Aman Baser by Aman Baser , 21, India

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4 solutions

Subhag Gupta
Nov 5, 2014

Natural Nos.= Starting from 1 till infinity. First 81 Natural Nos. means numbers till 81 including 1. Formula for sum of first 81 natural numbers- n/2[2a+(n-1)d] (It's Arithemetic Progression's Formula) (I am taking this Formula because the common difference between 2 consecutive terms =1. The Sum of AP= 1+2+3+4+........+81) For this question a=1(first term),d=1(common difference),n=81 81/2(2+80)=81/2*82=3321 So, 3321 is the sum. Now for the Average we'll divide it by 81- 3321/81=41 Simple.

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This is wrong. 0 is a natural number!

Cosmin Pletosu - 5 years, 1 month ago

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Oh yeah, the problem is wrong.

Ruilin Wang - 1 year, 10 months ago
Ruilin Wang
Jul 27, 2019

Just found that the answer is wrong. 0 is a natural number, reminded by @Cosmin Pletosu . The answer should be 40.

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0 exists on the number line but it is never used in counting objects. For instance ; When counting the number of cups on the shelf you don't start with 0,1,2 instead you say 1,2,3 naturally. 0 appears to be a neutral integer but it is not a natural number. This is an interesting discussion that is brought on https://study.com/academy/lesson/is-zero-a-natural-number.html

Gazar Khalid - 11 months, 1 week ago
Aakash Singh
Jan 6, 2015

sum of 1st n natural numbers is (n(n+1))/2 {using the A.P. sumation formula} now avg = (n(n+1))/2n = (n+1)/2 putting n=81 , avg=41

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Asim Das
Dec 18, 2014

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