I don't understand the math...

This isn't really a programming question, but a math one...

Dave says that this is the basic formula to generate a random number between an upper and a lower range:

Math.floor(Math.random() * (6 - 1 + 1)) + 1

First of all, he doesn't specify what range this covers (based on everyone else's solutions, I'm assuming that it's between 1 and 6)... but even so, 1) I'm still not sure how this would work with any other range, say between 3 and 5, and 2) how did he come to this solution?

For example, if we were to use the range 3 to 5:

Math.floor(Math.random() * (5 - 3 + 1)) + 1 Math.floor(Math.random() * (2 + 1)) + 1 Math.floor(Math.random() * (3)) + 1 <-- Essentially, we are just generating a random number from 1 to 3.... That doesn't cover 4 or 5 at all this way...