You have a working length of 16 mm and must leave at least 5 mm of gutta-percha apically. How much canal space is available for the post?

At least 5 mm of apical gutta-percha must remain to preserve the apical seal. 16 mm total minus 5 mm retained gutta-percha leaves 11 mm of space available for the post.Other options: 5, 7, and 8 mm would either remove too much apical seal or leave the post too short.Key exam takeaway: Retain at least 4-5 mm of apical gutta-percha; available post space = working length minus retained gutta-percha.