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Ratio Resolution III (Posted on 2012-12-14) Difficulty: 3 of 5
An alloy contains zinc and copper in the ratio 3 : 8 and another alloy contains zinc and copper in the ratio 7 : 15.
  1. When the two alloys are melted together in the ratio p:q, then the ratio of zinc and copper in the resulting alloy is p:q. Determine, with proof, the ratio p:q.
  2. What is the ratio p:q, if keeping all the other conditions in (i) unaltered, the ratio of zinc and copper in the resulting alloy is q:p?
Note: Assume that p and q does not have any common factor > 1.

  Submitted by K Sengupta    
Rating: 4.0000 (1 votes)
Solution: (Hide)
(i) By the problem:

The ratio of zinc and copper in the resulting alloy is
3p/11 + 7q/22
-------------
8p/11 + 15q/22

Accordingly by the given conditions, we must have:

 3p/11 + 7q/22     p
 -------------  = ---    ----(A)
 8p/11 + 15q/22    q

Applying  componendo to both sides of (A), we have:

  p + q           p + q
-------------  = -------  
8p/11 + 15q/22     q

Now p+q = 0 forces p = -q, which is a contradiction.

Consequently, 8p/11 + 15q/22 = q, giving:
8p/11 = 7q/22, so that:
p/q = (7*11)/(22*8) = 7/16

Therefore, the required ratio is 7:16
(ii) By the problem:

3p/11 + 7q/22       q
--------------- =  ----  ----- (B)
8p/11 + 15q/22      p 

Applying componendo to both sides of (B), we have:

  p + q            p + q
--------------  = -------  
8p/11 + 15q/22       p

Now p+q =0 forces p= -q, which is a contradiction.

Consequently, 8p/11 + 15q/22 = p, giving:
15q/22 = 3p/11, so that:
p/q = (15*11)/(22*3) = 5/2

Therefore, the required ratio is 5:2

Comments: ( You must be logged in to post comments.)
  Subject Author Date
alternate solutionJer2012-12-14 16:10:35
SolutionsolutionCharlie2012-12-14 12:42:26
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