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PyjamaScientist
If M and N are two positive integers such that 7 times M is 9 times N and 1260 is the least number that is a multiple of both M and N, then which among the following is the greatest number that can divide both M and N ?

(a) 20
(b) 30
(c) 40
(d) 50
(e) 60

7M = 9N = 1260. Since we cannot simplify down 7M = 9N, it must be equal to 1260 as that is the least common multiple.

M = 1260/7 = (1400 - 140)/7 = 200 - 20 = 180
N = 1260/9 = (900 + 360)/9 = 140.

The GCF of 180 and 140 is 20.

Ans: A
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chetan2u Would you like to share your 2 cents on this. Here, I mistook N as M and vice versa. It does not make any difference to the final answer though. So, kindly ignore that.
Also, since I have taken, 7N = 9M. I concluded that N will be a multiple of 9 (since 7 isn't) and similarly M will be a multiple of 7 (since 9 isn't) and then continued with calculating the max values for both through the LCM given 1260. This 1260 gives me maximum powers that prime factors of either of M or N can take. So, maximizing powers for max value of M and N, along with the nature of N (=9k) and M (=7K), made me come to this final solution. How would you have done it?
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chetan2u Could you please tell me if I have done it right?

7M = 9N
M/N = 9/7

Product of two integers = Product of their LCM & HCF

M = 9k
N = 7k
LCM = 1260
HCF = x

M * N = 1260 * HCF
9*7*k*k = 1260 * x
x = 9*7*k^2/1260

x = k^2/20 (A)
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bv8562
chetan2u Could you please tell me if I have done it right?

7M = 9N
M/N = 9/7

Product of two integers = Product of their LCM & HCF

M = 9k
N = 7k
LCM = 1260
HCF = x

M * N = 1260 * HCF
9*7*k*k = 1260 * x
x = 9*7*k^2/1260

x = k^2/20 (A)

You are absolutely correct.
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­7M = 9N ---> Derived LCM(7,9) = 63

Actual LCM(M,N) = 1260

HCF = Actual LCM/Derived LCM = 1260/63 = 20 (option A)

Kindly confirm if there is any fault in this approach. Thanks.
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