Bob has to bring $4$ dozens of pens to school. Among $10$ pens he already has, $30\%$ of them are blue pens and the rest of them are black pens. Bob wanted to havehalf of black pens and half of blue pens out of total $4$ dozens. How many blue pens does he have to buy? A. $21$A. $21$ B. $38$ C. $17$ D. $45$ E. $3$

Show AnswerShow The First Step1. First, Bob needs to know how many blue pens he already has. Because there are $30\%$ of $10$ blue pens, he can get the number of blue pens by calculating $\displaystyle \frac{30}{100} \times 10 = 3$.Show Next Step2. Remember $4$ dozen is $48$. Because he wanted to have a half of blue pens out of $48$, the number of blue pens to bring to school is $24$. Bob already has $3$ number of blue pens. So, he needs to buy ${24} - {3} = 21 $ number of blue pens. No More Steps