If you find the sum of the squares of the first n natural numbers, which n gives you a sum of 1240?
12
13
15
18
Explanation:
Sum of squares of n numbers is given by the formula $\dfrac{n × (n + 1) (2n + 1)}{6}$
Plug in each of the options and find the sum which = 1240
n = 12; S12 = [12 × (12+1) (2×12 + 1)]/6 = 650
Since n = 12 gives too low a value than 1250, skip n = 13 and try n = 15.
= [15 × 16 × 31] /6
= 1240