Pencils and Pens Problem

Sarah has a collection of pens and pencils. The number of pens she has is represented by the variable P, and the number of pencils is represented by the variable Q. According to her calculations, the total number of writing instruments she has is 30. Therefore, we can express this situation using the equation:

$$ P + Q = 30 $$

If Sarah decides to buy 10 more pens, how many pencils will she have left?