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Math Help - Help about ratio and proportion

  1. #1
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    Help about ratio and proportion

    The ratio between the incomes of two people is 5:3. The ratio between their expenditures is 8:5, while the ratio between their savings is 4:3. What will be the ratio between their combined Incomes and Combined Expenditures. (Answer = 32:39) Please tell me how to do it.
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  2. #2
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    Define:
    [Math]I_1,I_2[/tex] Incomes of person 1 and 2
    [Math]E_1,E_2[/tex] Expenditure of persons 1 and 2.


    You are given the following ratios:
    \frac{I_1}{I_2} = \frac{5}{3}

    \frac{E_1}{E_2} = \frac{8}{5}

    \frac{I_1 - E_1}{I_2 - E_2} = \frac{4}{3}

    This is 3 equations in 4 unknowns, so we can choose one of the values. I will set I_1 = 1

    Now:
    \frac{1}{I_2} = \frac{5}{3}

    \frac{E_1}{E_2} = \frac{8}{5}

    \frac{I_1 - E_1}{I_2 - E_2} = \frac{4}{3}

    You should be able to solve that system of simultaneous equations to get
    I_1 = 1
    I_2 = 0.6
    E_1 = 1.2
    E_2 = 0.75

    The ratio you want is

    \frac{I_1 + E_1}{I_2 + E_2} = \frac{1.6}{1.95} = \frac{32}{39}


    Its interesting to note that the savings amount comes out as negative for both people...but thats another story.


    If you are interested in why i can set I_1 = 1 without breaking the answer, its because we are only interested in ratios, not the actual levels of income. I am effectively redefining the currency unit of the problem so that I_1 has 1 unit.
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  3. #3
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    Quote Originally Posted by navikhi View Post
    The ratio between the incomes of two people is 5:3. The ratio between their expenditures is 8:5, while the ratio between their savings is 4:3. What will be the ratio between their combined Incomes and Combined Expenditures. (Answer = 32:39) Please tell me how to do it.
    \frac{I_A}{I_B}=\frac{5}{3}\ \Rightarrow\ I_B=\frac{3}{5}I_A

    \frac{E_A}{E_B}=\frac{8}{5}\ \Rightarrow\ E_B=\frac{5}{8}E_A

    Therefore

    \frac{I_A+I_B}{E_A+E_B}=\frac{I_A+\frac{3}{5}I_A}{  E_A+\frac{5}{8}E_A}=\frac{\frac{8}{5}I_A}{\frac{13  }{8}E_A}

    =\frac{I_A}{E_A}\ \frac{8}{5}\ \frac{8}{13}=\frac{64}{65}\ \frac{I_A}{E_A}

    We can obtain the relationship between I_A\ and\ E_A from the savings ratio

    \frac{S_A}{S_B}=\frac{I_A-E_A}{I_B-E_B}=\frac{4}{3}

    \frac{I_A-E_A}{\frac{3}{5}I_A-\frac{5}{8}E_A}=\frac{4}{3}

    3I_A-3E_A=4\left(\frac{3}{5}I_A\right)-4\left(\frac{5}{8}\right)E_A

    3I_A-3E_A=\frac{12}{5}I_A-\frac{20}{8}E_A

    \left(\frac{15}{5}-\frac{12}{5}\right)I_A=\left(\frac{24}{8}-\frac{20}{8}\right)E_A

    \frac{3}{5}I_A=\frac{4}{8}E_A

    24I_A=20E_A

    E_A=\frac{24}{20}I_A

    Expenditure exceeds income, hence the savings is <0

    Therefore we can use the above to calculate the required ratio

    \frac{64}{65}\ \frac{I_A}{E_A}=\frac{64}{65}\ \frac{I_A}{\frac{24}{20}I_A}

    =\frac{64}{65}\ \frac{20}{24}=\frac{64}{65}\ \frac{5}{6}=\frac{32}{3}\ \frac{5}{65}=\frac{32}{3}\ \frac{1}{13}

    =\frac{32}{39}
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