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If $ \frac { 1 }{ 2 } a + \frac { 3 }{ 4 } b = 10 $, and $ k\left( \frac { 1 }{ 2 } a + \frac { 3 }{ 4 } b \right) =15 $, then what is the value of the constant $ k $?

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$ 5 $

$ \cfrac { 3 }{ 2 } $

$ 2 $

$ \cfrac { 1 }{ 2 } $