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Ratio and proportion - HEEEEELLLLPP!

  1. May 20, 2009 #1
    Hi!
    I dont have ANY IDEA on how to do ANY kind of math problem coz i am not a math student but i have to do some QA questions for my entrance exams so I need a little bit of help here!
    so...here's the question:-

    The monthly incomes of A and B are in the ratio 8:7 and those of B and C are in the ratio 5:3. If the monthly income of C is Rs.3,360, find the monthly income of A.

    Options:
    a) Rs.3500
    b) Rs.4200
    c) Rs.5600
    d) Rs.6400
    I tried solving the problem and I can't get the answer in more than 3 digits! Desperately need help here!
     
  2. jcsd
  3. May 20, 2009 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    A ratio is a fraction and proportion is a statement that two fractions are equal. Here you are told that A/B= 8/7 and that B/C= 5/3. From the first equation, multiplying both sides by 7B, 7A= 8B. Dividing both sides by 8, B= (7/8)A. Doing the same with the second equation, 3B= 5C, B= (5/3)C. So B= (7/8)A= (5/3)C. Multiply both sides by 8/7 to get A= (8/7)(5/3)C= (40/21)C. Since C= 3,360, A= (40/21)(3360). I don't understand what you mean by "more than 3 digits". It just simple arithmetic. Are you allowed to use a calculator? Even without a calculator it is easy to see that 3360 is divisible by 3: 3360/3= 1120 so 3360/21= 1120/7 and, remarkably, 1120/7= 160. Can you multiply 40(160)?

    And this surely does not belong in "Calculus and Beyond". It is, as I said, simply arithmetic.
     
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