Chairs Needed for School Event

Myra is organizing a school event and needs to set up chairs. She needs a total of 50 chairs for the event.

The number of chairs she already has, represented by $c$, plus the additional chairs she needs to buy, represented by $b$, can be expressed by the equation:

$$ c + b = 50 $$

If Myra currently has 20 chairs, how many additional chairs does she need to buy?