what does the below math rebus puzzle mean?

Take one out and scratch my head, I am now black but once was red. What am I?

Three friends named Mr. Black, Mr. Yellow and Mr. Green enter a pub on a weekend night. They are wearing either a Black, Yellow or Green shirt.

Mr. Yellow says to them, 'Did you notice that we all are wearing different color shirts than our names?'

To this, the man who was wearing the Green shirt said. 'Wow, thats right.'

Can you identify who is wearing which colour shirt ?

I had an infinite supply of water and 5 litres and 3 litres jars.

How would you measure exactly 4 litres in the least number of steps?

What has a ring but no finger?

I don't get anymore wetter, no matter how much rain falls on me.

In a rainy season, one day 3 huge ladies walked under a normal-sized umbrella.
The umbrella is not huge enough to accommodate all 3 ladies yet not a single lady got wet. Why?

I shave every day, but my beard stays the same. What am I?

I have cities, but no houses. I have mountains, but no trees. I have water, but no fish. What am I?

Irene killed hundreds of people mercilessly and yet the police were not able to put her behind the bars. Why?