KVPY Sample Paper KVPY Stream-SX Model Paper-26

  • question_answer
    Initially there is 50 gm of salt in tank with 100 L of water present. A liquid at rate of 5 L/min with 2 gm/L of salt is coming into tank. After proper mixing in tank it is running out with 4l/min. The amount of salt present in tank after time t=100 min is

    A) \[300gm\]

    B) \[\frac{1525}{4}gm\]

    C) \[\frac{1555}{4}gm\]

    D) \[\frac{3125}{8}gm\]

    Correct Answer: D

    Solution :

    Let the amount of salt present in tank is ?m? gm
    \[\frac{dm}{dt}=10-{{\left( \frac{m}{100+t} \right)}^{4}}\]\[\Rightarrow \frac{dm}{dt}+\frac{4m}{100+t}=10\] \[\Rightarrow {{\left( 100+t \right)}^{4}}m=\frac{10{{(100+t)}^{5}}}{5}+c\] \[\Rightarrow m{{\left( 100+t \right)}^{4}}=2{{\left( 100+t \right)}^{5}}+c\]At \[t=0,{{m}_{0}}=50gm\]
    \[50{{\left( 100 \right)}^{4}}=2{{\left( 100 \right)}^{5}}+c\]
    \[c=-150\times {{100}^{4}}\]
    \[\therefore {{m}_{t}}=\frac{2{{\left( 100+t \right)}^{5}}}{{{\left( 100+t \right)}^{4}}}-\frac{150\times {{100}^{4}}}{{{\left( 100+t \right)}^{4}}}\]
    \[\therefore {{m}_{100}}=2\left( 100+100 \right)-\frac{150\times {{100}^{4}}}{{{\left( 200 \right)}^{4}}}\]
    \[{{m}_{100}}=400-\frac{150}{16}\]
    \[{{m}_{100}}=400-\frac{75}{8}=\frac{3200-75}{8}=\frac{3125}{8}\]


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