A bunch of guys drive to a river chanting about how they are going to ride a "level 5 rapid". When they finally get there and see how wild the water is, they stare at each other and one finally says, "If we take a level 3 rapid and then a level 2 rapid, it adds up to a level 5, no?". The others then say, "Uh, yeah, I think you are right. Let's go....."

Thus, if you have 1.5 years of version 6 and 1.5 years of version 7, then it adds up to 3 years of version 6.5.

Anyhow, the job probably goes to the best liar.