Some consecutive natural numbers, starting with 1, are written on the board. Now, one of the numbers was erased and the average of the remaining numbers is 800/39. Find the number which was erased.  Choose one
A) 20
B) 87
C) 266
D) 288
E) 11

Rationale: we know that average of n consecutive numbes average = n × ( n + 1 ) 2 n = ( n + 1 ) 2 n × ( n + 1 ) 2 n = ( n + 1 ) 2 if the given n is sufficiently large , the average does not change much even though we exclude one or two numbers from it . so the approximate number of observations is almost double to the average ( remember : the average of consecutive numbers almost lies in the middle ) the approximate average is 800 / 39 = approx 20 . so the initial numbers may be nearer to 40 . in this question it is actually 40 as from the denominator of the new average 800 / 39 . the initial numbers are 40 . sum of 40 consecutive numbers = 40 × ( 40 + 1 ) 2 = 82040 × ( 40 + 1 ) 2 = 820 sum of 39 numbers = average x numbenumber of observations = 80039 × 3980039 × 39 = 800 so the number excluded = 820 - 800 = 20 answer : a
Answer: A