Question

The sum of three consecutive odd numbers is 69 find prime no. out of these

Answer 1

let the first odd no. be 2n+1

second be 2n+3

third be 2n+5

Their sum = (2n+1) + (2n+3) + (2n+5) = 69

or, 6n+9 = 69

or, 6n = 69 - 9 = 60

so, n = 60/6 = 10

Hence the nos. are : 2(10)+1 , 2(10)+3 , 2(10)+5

i.e., 21, 23, 25

out of these clearly 23 is a prime no. which is only divisible by 1 and 23 only.