Which of the following numbers is not the sum of three consecutive integers: 51, 61, 72, 81?
One of the properties of any three consecutive numbers is that their sum is always a multiple of 3.
Why is this?
Consecutive numbers can be written as
The sum of 3 consecutive numbers is given by
Which of the given numbers are divisible by 3?
You can simply add their digits to find out.
If the sum of the digits of a number is a multiple of 3, then the number itself is divisible by 3.
Only 61 is not divisible by 3. Therefore it is not the sum of three consecutive numbers.