Last visit was: 13 Dec 2024, 22:05 It is currently 13 Dec 2024, 22:05
Close
GMAT Club Daily Prep
Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track
Your Progress

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History
Not interested in getting valuable practice questions and articles delivered to your email? No problem, unsubscribe here.
Close
Request Expert Reply
Confirm Cancel
User avatar
Bunuel
User avatar
Math Expert
Joined: 02 Sep 2009
Last visit: 13 Dec 2024
Posts: 97,874
Own Kudos:
685,647
 []
Given Kudos: 88,269
Products:
Expert reply
Active GMAT Club Expert! Tag them with @ followed by their username for a faster response.
Posts: 97,874
Kudos: 685,647
 []
Kudos
Add Kudos
11
Bookmarks
Bookmark this Post
User avatar
nick1816
User avatar
Retired Moderator
Joined: 19 Oct 2018
Last visit: 12 Dec 2024
Posts: 1,859
Own Kudos:
7,095
 []
Given Kudos: 707
Location: India
Posts: 1,859
Kudos: 7,095
 []
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
avatar
nik021
Joined: 05 Oct 2017
Last visit: 07 Aug 2019
Posts: 10
Own Kudos:
Given Kudos: 1,074
Posts: 10
Kudos: 2
Kudos
Add Kudos
Bookmarks
Bookmark this Post
User avatar
nick1816
User avatar
Retired Moderator
Joined: 19 Oct 2018
Last visit: 12 Dec 2024
Posts: 1,859
Own Kudos:
7,095
 []
Given Kudos: 707
Location: India
Posts: 1,859
Kudos: 7,095
 []
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
In question they asked for ordered pair, hence (a,b) is different from (b,a)
Now if you put the value of x and y, you will get 2 solutions
(a,b)= (21, 189) and (189, 21).

nik021
nick1816
a*b= lcm(a,b) * gcd(a,b)
Let lcm(a,b)= l
and gcd(a.b)=g
Hence we can re-write the equation as
lg+63=20l+12g
or l=(12g-63)/g-20
or l= 12+(177/g-20)
l must be a positive integer, hence 177/g-20 will also an integer
g-20 can take 8 values -177, -59, -3, -1,1,3,59 or 177
check for each value, it must satisfy 2 conditions
1. l and g must be positive
2. l is a multiple of g

Only case satisfied both conditions when g-20=1, g=21
l= 12+177/1=189

gcd(a,b)=21
a=21x and b=21y

lcm(a,b)=189
only possible when either x=1 and y=9, or when x=9 and y=1
Hence 2 solutions are possible



the solution is complicated and do not justify the answer!
does anybody have better solution for this?
User avatar
Kinshook
User avatar
GMAT Club Legend
Joined: 03 Jun 2019
Last visit: 13 Dec 2024
Posts: 5,423
Own Kudos:
4,599
 []
Given Kudos: 161
Location: India
GMAT 1: 690 Q50 V34
WE:Engineering (Transportation)
Products:
GMAT 1: 690 Q50 V34
Posts: 5,423
Kudos: 4,599
 []
1
Kudos
Add Kudos
Bookmarks
Bookmark this Post
Bunuel
How many ordered pairs (a, b) of positive integers satisfy the equation a*b + 63 = 20*lcm}(a, b) + 12*gcd(a,b), where gcd(a,b) denotes the greatest common divisor of a and b, and lcm(a,b) denotes their least common multiple?

A. 0
B. 2
C. 4
D. 6
E. 8

Asked: How many ordered pairs (a, b) of positive integers satisfy the equation a*b + 63 = 20*lcm}(a, b) + 12*gcd(a,b), where gcd(a,b) denotes the greatest common divisor of a and b, and lcm(a,b) denotes their least common multiple?

Let the lcm(a, b) be l and gcd(a, b) be g.
ab = lg

lg + 63 = 20l + 12g
l (g-20) = 12g - 63
l = (12g-63)/(g-20) = 12 + 177/(g-20)
l must be a multiple of g and l>g
g = 21 = 3*7; l = 189= 3*3*3*7 = 3^3*7

(a,b) = {(21,189),(189,21)}

2 solutions are possible

IMO B
Moderator:
Math Expert
97874 posts