-
Dinen
- Junior | Next Rank: 30 Posts
- Posts: 16
- Joined: Fri Jul 02, 2010 4:22 am
- Location: Cape Town, South Africa
- Thanked: 1 times
Hi all....here goes my first post on this forum. My query refers to DS problem 10, Chapter 10 (p.133) Mgmat number properties:
Question states:
"Is the sum of integers a and b divisible by 7?
1) a is not divisible by 7.
2) a-b is divisible by 7.
Herewith my interpretation: 1) Obviously insufficient
2) If a-b is divisible by 7, hence a-b is a multiple of 7. Rule states that when adding/subtracting a mulitple of say N, to a non-multiple of N, one would always get a multiple of N as the answer. Apply this to the above, statement 2 tells me either a or b is a multiple of seven and the other is not. Extrapolating this arguement , I can conclude the same for a + b and hence say sufficient ie. answer B.
However answer is C (both statements required). Even tho' they give numerical examples illustrating why 2 is insufficient (ie a, b = 21, 14 & a,b = 20, 13) and go on with a remainder explanation that I'm still trying to wrap my head around, the fundamentalist in me still requires someone to point out why my reasoning alluded to above is incorrect! Input will sincerely be appreciated....ta in advance.
[/i]
Question states:
"Is the sum of integers a and b divisible by 7?
1) a is not divisible by 7.
2) a-b is divisible by 7.
Herewith my interpretation: 1) Obviously insufficient
2) If a-b is divisible by 7, hence a-b is a multiple of 7. Rule states that when adding/subtracting a mulitple of say N, to a non-multiple of N, one would always get a multiple of N as the answer. Apply this to the above, statement 2 tells me either a or b is a multiple of seven and the other is not. Extrapolating this arguement , I can conclude the same for a + b and hence say sufficient ie. answer B.
However answer is C (both statements required). Even tho' they give numerical examples illustrating why 2 is insufficient (ie a, b = 21, 14 & a,b = 20, 13) and go on with a remainder explanation that I'm still trying to wrap my head around, the fundamentalist in me still requires someone to point out why my reasoning alluded to above is incorrect! Input will sincerely be appreciated....ta in advance.
[/i]
You never too old to grow up
