A student finds the sum 1 + 2 + 3 + ... as his patience runs out. He found the sum as 575. When the teacher declared the result wrong, the student realized that he missed a number. What was the number the student missed?  Choose the correct choice from the following
A) 16 B) 18 C) 14 D) 20 E) 22

Rationale: explanation : sum of 1 st 10 natural numbers ( 1 – 10 ) = 1 + 2 + … 10 = 55 . sum of next 10 natural numbers ( 11 – 20 ) = 10 + 11 + 12 + … 20 = 100 + 55 = 155 . sum of next 10 natural numbers ( 31 – 40 ) = 255 . sum of numbers from 31 – 40 = 355 and so on and on . so , we have here 55 + 155 + 255 = 465 . if we add 355 to 465 , it will be way more than 575 . so , we can add one by one . 465 + 31 + 32 + 33 = 561 . 465 + 31 + 32 + 33 + 34 = 595 . thus , the sum should be 595 instead of 575 , and the missing number is 20 . answer : d
Answer: D