What is the next number in this matchsticks series riddle?

Who has married many women but was never married?

Can you solve this picture maths equation?

Name a key which is hardest to turn.

It's a protector.
It sits on a bridge.
One individual can see directly through it, while others wonder what it hides.

What is it?

They are three errirs in this question. Can you find them?

I have a hole in my back and legs I lack.
I live where I can’t breathe and I eat without teeth.
What am I?

John was a very careless driver, so his owner Jacob gave him an offer that he will get an incentive of Rs.30 for every bottle box he delivered without breaking it and he will be charged Rs.90 for every bottle box he broke. Jacob gave John 100 bottles-box to deliver. After delivery, Jacob paid John Rs.2400. How many bottles-box did John break?

what does the below math rebus puzzle mean?

A Scientist was fifty years old in the year 2000 but he was forty years old in the year 2010. How can this be possible?

PS: It has a logical answer.