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?
Sum of first $$'n'$$ natural numbers = $$\frac{n(n+1)}{2}$$
=> Sum of first 12 natural numbers = $$\frac{12(12+1)}{2}=78$$
Thus, number that was added twice = $$80-78=2$$
=> Ans - (B)
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