Hey can anybody help !!!!
IF n and y are positive integers and 450y=n^3, which of the following must be an integer?
I. y/3x2^2x5
II. y/3^2x2x5
III. y/3x2x5^2
Reminder x is multiplication...
A. NONE
B. I only
C. II only
D. III only
E. I, II, and III
GMAT QUES?
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 195
- Joined: Sun Oct 21, 2007 4:33 am
- Thanked: 10 times
Solve using prime factors
450y = 3 x 3 x 5 x 5 x 2 x (y) = n^3
Therefore y must contain atleast (5 x 3 x 2 x 2)
I .y/3x2^2x5 - YES (Denominator has 5 x 3 x 2 x 2)
II. y/3^2x2x5 - NO (Denominator only has one 2, we need 2 x 2)
III. y/3x2x5^2 - NO (Same as II)
Option B
450y = 3 x 3 x 5 x 5 x 2 x (y) = n^3
Therefore y must contain atleast (5 x 3 x 2 x 2)
I .y/3x2^2x5 - YES (Denominator has 5 x 3 x 2 x 2)
II. y/3^2x2x5 - NO (Denominator only has one 2, we need 2 x 2)
III. y/3x2x5^2 - NO (Same as II)
Option B
-
- Master | Next Rank: 500 Posts
- Posts: 195
- Joined: Sun Oct 21, 2007 4:33 am
- Thanked: 10 times
General rule: x^3 has three x's
Similarly 450 has 3x3, 5x5 and 2. So, if 450y = n^3 it needs one 3, one 5 and two 2's to (i.e. 3x5x2x2) which should come from Y
Another perspective...
If y = 3 x 5 x 2 x 2 then,
450y= 3 x 3 x 5 x 5 x 2 x 3 x 5 x 2 x 2 (each number is present 3 times) and will be a perfect cube.
Hope that explains!
Similarly 450 has 3x3, 5x5 and 2. So, if 450y = n^3 it needs one 3, one 5 and two 2's to (i.e. 3x5x2x2) which should come from Y
Another perspective...
If y = 3 x 5 x 2 x 2 then,
450y= 3 x 3 x 5 x 5 x 2 x 3 x 5 x 2 x 2 (each number is present 3 times) and will be a perfect cube.
Hope that explains!