The average

The average of one set of 4 numbers is 35. The average of another set of number is 20. The average of the numbers in the two sets is 30. How many numbers are there in the other set?

Correct result:

n =  2

Solution:

$$\begin{gathered} {\displaystyle \frac{4\cdot 35+20\cdot n}{4+n}}=30 \\ 4\cdot 35+20\cdot n=30\cdot (4+n) \\ 10n=20 \\ n=2 \end{gathered}$$

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