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Keerthana
Posted on

A boy added all natural numbers from 1 to 12, however he added one number twice due to which the sum becomes 80. What is the number which he added twice?

a

3

b

8

c

7

d

2

Answer : Option D
Explanation :

1+2+3+…+n=n(n+1)21+2+3+ldots+n=frac{n(n+1)}{2}

∴1+2+3+…+12=12×132=78 herefore 1+2+3+ldots+12=frac{12 imes 13}{2}=78

∴ Required number = 80 – 78 = 2

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