What's the average weight of a hiker's back pack?

If you were going hiking in hilly terrain..even mountainous and were planning on carrying all your basic camping gear clothes, supplies etc what would the average weight be do you think?

I’m thinking of what an adult female would be carrying for herself and perhaps one child (so a two man tent) and supplies for a 4 days trek.

Actual weights from experienced hikers most welcome and just plain guesses also!