I am preparing for an English language test. That is why I am working on strengthening my English vocabulary.
My question is regarding the speaking section of the test.
If the examiner asks me about a particular subject, for example mobile development, would it be too informal to say "it's my bread and butter"?
If yes, then could you please suggest to me a word for that? I don't want to write a sentence like "that is how I make money or that is the source of my income"
PS: If you find any grammatical or spelling mistakes in this question then please do let me know. I would appreciate that.