A 20\textrm{ ft}\times 10\textrm{ ft} stage is bordered on three sides by "backstage" and "wing" space that is x feet wide. The overall dimensions of the combined lot and walkway are 20 \textrm{ ft}\times 10 \textrm{ ft}. The combined area is 450\textrm{ ft}^{2}. What is the value of x?