Question : A boy added all natural numbers from 1 to 12, but he added one number twice, due to which the sum became 80. What is the number that he added twice?
Option 1: 3
Option 2: 2
Option 3: 7
Option 4: 8
Correct Answer: 2
Solution : Sum of the first $n$ natural numbers = $\frac{n(n + 1)}{2}$ Sum of the first 12 natural numbers = $\frac{12(12 + 1)}{2}$ = $\frac{12\times 13}{2}$ = 78 A number is added twice so that the sum becomes 80. The number added twice is (80 – 78) = 2 Hence, the correct answer is 2.
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Question : A boy added all-natural numbers from 1 to 20. However, he added one number twice, due to which the sum became 215. What is the number which he added twice?
Option 1: 5
Option 2: 7
Option 3: 11
Option 4: 15
Question : The sum of three consecutive natural numbers divisible by 3 is 45. The smallest number is:
Option 1: 18
Option 2: 3
Option 3: 12
Option 4: 9
Question : Which one of the following is a factor of the sum of the first twenty-five natural numbers?
Option 1: 26
Option 2: 24
Option 3: 13
Option 4: 12
Question : The product of two numbers is 24 times the difference between these two numbers. If the sum of these numbers is 14, the larger number is:
Option 1: 9
Option 2: 8
Option 4: 10
Question : The LCM of four consecutive numbers is 60. The sum of the first two numbers is equal to the fourth number. What is the sum of the four numbers?
Option 1: 17
Option 2: 14
Option 3: 21
Option 4: 24
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