enigma123 wrote:
What is the positive integer n?
(1) For every positive integer m, the product m(m + 1)(m + 2) ... (m + n) is divisible by 16
(2) n^2 - 9n + 20 = 0
Guys - the OA is C. But can someone please let me know what does statement 1 implies over here?
For me it says that the product of consecutive integers is divisible by 16. But how its used in this question?
I understand statement 2 though.
Product of consecutive integers have some properties. e.g. product of any two consecutive positive integers is even, product of any 3 consecutive integers is even and is divisible by 3 so basically the product is divisible by 6.
The first statement just tells you that every product of (n+1) consecutive integers is always divisible by 16.
Say if n = 3
Is 1*2*3*4 divisible by 16? No! So n cannot be 3.
Is 1*2*3....*14*15*16 divisible by 16? Yes. Will product of any 16 consecutive integers be divisible by 16? Yes. Product of any 17 consecutive integers will also be divisible by 16. So n can take many values.
What is the smallest value that n can take?
Every set of 4 consecutive integers will have a number which has 4 as a factor and it will have another even number i.e. the product of 4 consecutive numbers must be divisible by 8.
When you have product of 5 consecutive factors, again the product must be divisible by at least 8 e.g. 1*2*3*4*5
When you have 6 consecutive factors, there must be a number with 4 as a factor and 2 other even numbers i.e. the product must have 16 as a factor e.g. 1*2*3*4*5*6
Therefore, you must have at least 6 consecutive integers i.e. n must be at least 5.
fameatop: n cannot be 4. If you have 5 consecutive factors, the product will not be divisible by 16 in every case e.g. it is not divisible in case of 1*2*3*4*5
Statement 1 tells you that n is at least 5
Statement 2 tells you that n is 4 or 5.
So n must be 5 using both
Answer (C)