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A boy added all natural numbers from 1 to 20. However he added one number twice, due to which the sum becomes 215. What is the number which he added twice?
- A5
- B7
- C11
- D15
Answer not set
Discussion 1
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CCommunity
Let, the number he added twice = x
Sum of all natural numbers from 1 to 20,
⇒ 20 × (20 + 1)/2
⇒ 10 × 21
⇒ 210
According to problem,
⇒ 210 + x = 215
⇒ x = 5
Sum of all natural numbers from 1 to 20,
⇒ 20 × (20 + 1)/2
⇒ 10 × 21
⇒ 210
According to problem,
⇒ 210 + x = 215
⇒ x = 5