Classs mean

The class size is 30 students. Math was not worse than mark 2. Find the number of pupils who were marked one if the class had a 1.4 average mark.

Correct answer:

a =  18

Step-by-step explanation:

$$\begin{gathered} n=30 \\ a+b=n \\ \frac{1a+2b}{n}=1.4 \\ a+2b=1.4\cdot n \\ a+2(n-a)=1.4\cdot n \\ a+2(30-a)=1.4\cdot 30 \\ a+2\cdot (30-a)=1.4\cdot 30 \\ a=18 \\ a=\frac{18}{1}=18 \\ a=18 \\ b=n-a=30-18=12 \end{gathered}$$

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