Since your question is "What am I supposed to tell them?", I'm going to go a little bit off the literal math with my answer. What you should say is:

*The more "times-zoom" a lens has, the more compromise on image quality it has to have. Since my camera allows me to change lenses, I can have the flexibility of a huge zoom range without compromising on the results.*

Then, I'd steer the conversation this way:

*In fact, some very popular lenses for SLR cameras have no zoom at all, because they can be high quality and let in a lot of light without being expensive and gigantic. On the other hand, some lenses like my 18-135mm have a relatively high zoom ratio, allowing you to choose that convenience when you want it.*

At that point, they may just smile and nod and back away. But they may also learn something. :)