1 Answers
Option 2 : 18th person
⇒ The donation made by everyone is in the form of squares of integers.
Assume that donation made by nth person surpasses the tally of 2000 Rs.
⇒ 12 + 22 + 32 + 42 + ......... + n2 > 2000
⇒ n/6 × (n + 1)(2n + 1) > 2000
By Hit and Trial,
⇒ Try for n = 17
n/6 × (n + 1)(2n + 1) = 17/6 × (18) (35) = 1785 < 2000
⇒ Now, try n = 18
n/6 × (n + 1)(2n + 1) = 18/6 × (19)(37) = 2109 > 2000
Hence, donation made by 18th person will surpass the tally of Rs. 2000.
4 views
Answered