1.

1). 52). 73). 114). 15

Answer»

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



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