enigma123 wrote:
The sum of n consecutive positive integers is 45. What is the value of n?
(1) n is even
(2) n < 9
Official Solution
Credit: Veritas Prep
Solution: First I will give the solution of this question and then discuss the logic used to solve it.
In how many ways can you write n consecutive integers such that their sum is 45? Let’s see whether we can get such numbers for some values of n.
n = 1 -> Numbers: 45
n = 2 -> Numbers: 22 + 23 = 45
n = 3 -> Numbers: 14 + 15 + 16 = 45
n = 4 -> No such numbers
n = 5 -> Numbers: 7 + 8 + 9 + 10 + 11 = 45
n = 6 -> Numbers: 5 + 6 + 7 + 8 + 9 + 10 = 45
Let’s stop right here.
Statement I: n must be even.
n could be 2 or 6. Statement I alone is not sufficient.
Statement II: n < 9
n can take many values less than 9 hence statement 2 alone is not sufficient.
Both statements together: Since n can take values 2 or 6 which are even and less than 9, both statements together are not sufficient.
Logic
Now, the interesting thing is how do we get these numbers for different values of n. How do we know the values that n can take? It’s pretty easy really. Follow my thought here.
Of course, n can be 1. In that case we have only one number i.e. 45.
n can be 2. Why? When we divide 45 by 2, we get 22.5. Since 2*22.5 is 45, we have to find 2 consecutive integers such that their arithmetic mean is 22.5. The integers are obviously 22 and 23.
n can be 3. When we divide 45 by 3, we get 15. So we need 3 consecutive integers such that their mean is 15. They are 14, 15, 16.
When we divide 45 by 4, we get 11.25. Do we have 4 consecutive integers such that their mean is 11.25? No, because mean of even number of consecutive integers is always of the form x.5.
n can be 5. When we divide 45 by 5, we get 9 so we need 5 consecutive integers such that their mean is 9. They must be 7, 8, 9, 10, 11.
n can be 6. When we divide 45 by 6, we get 7.5. We need 6 consecutive integers such that their mean is 7.5. The integers are 5, 6, 7, 8, 9, 10
Obviously, we just need to focus on getting 2 even values of n which are less than 9. So we check for 2, 4 and 6 and we immediately know that the answer is (E). We don’t have to do this process for all numbers less than 9 and we don’t have to do it for odd values of n.