In a museum

There are 250 people in a museum
2/5 of the 250 people are girls.
3/10 out of the 250 people are boys.
The rest of the 250 people are adults.
Find the number of adults at the museum.

Correct answer:

a =  75

Step-by-step explanation:

$$\begin{gathered} n=250 \\ g=\frac{2}{5}\cdot n=\frac{2}{5}\cdot 250=\frac{2\cdot 250}{5}=\frac{500}{5}=100 \\ b=\frac{3}{10}\cdot n=\frac{3}{10}\cdot 250=\frac{3\cdot 250}{10}=\frac{750}{10}=75 \\ b+g+a=n=250 \\ a=n-b-g=250-75-100=75 \end{gathered}$$

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