I mean lets assume I have 5 zoos - 4 zoos, how many zoos are left, 1 zoo .. but once the question talks about bits in computing world, then for example 20 bits - 4bits, how many bits left? I sometime get stuck three hours on that simply question .... and I think that's because I don't know what subtract means, as a result I must first understand what's the term subtract mean in order to get used by it and manipulate it by problems/question.

So may anyone please explain to me what's subtract means? dont tell me it means subtract two quantity .. I need something more senseable .. thanks alot