I have a table that has the vapor pressure of water at selected temperatures. It is converted from celsius to torr. For example:

Question

18°C = 15.5 torr
19°C = 16.5 torr

Unfortunately, the temperature I got is 18.5°C. How do I convert it to torr? The book has no equation and I can't find it anyhwere.

The best thing to do is to interpolate.
You want 18.5 which is half way between 18 and 19. So take half way between 15.5 and 16.5. For small differences like this there is little error involved.