A boy added all natural numbers from 1 to 10, however he added one number twice due to which the sum becomes 58. What is the number which he added twice?

Solution (By Examveda Team)

Sum of natural number from 1 to 10
$$\eqalign{ & = \frac{{\left( n \right)\left( {n + 1} \right)}}{2}\,\,\,\,\,\,\,\left( {{\text{where }}n = 10} \right) \cr & = \frac{{10 \times 11}}{2} \cr & = 55 \cr} $$
Clearly, we can see the number which is added twice = 58 - 55 = 3