Ten pupils

10 pupils came to the art group. 8 pupils painted with watercolors and 9 pupils painted with ink, each pupil painted with ink or watercolors. How many pupils painted water and ink at the same time?

Correct result:

x =  7

Solution:

$$\begin{gathered} (8-x)+x+(9-x)=10 \\ x=7 \end{gathered}$$

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