Re: pants for working in
The "in" is the equivalent of "[while] wearing".
"They are the trousers that I work while wearing." I should point out that that makes a rather unnatural sentence and would normally be said "They are the trousers that I wear while working".
We use "in" a lot when referring to clothes:
- It was so hot yesterday, I went out in only a T-shirt and shorts.
- She was seen at work in jeans, a diamanté tiara and stilettos.
- She walked down the red carpet in a red Gucci dress.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.