The minus sign outside the brackets negates or reverses the minus sign in the brackets, making it plus, in a similar way to language, where a double negative statement becomes a positive statement. Another way to look at positives and negatives is to think of direction: positive means move forward and negative means move back. So one negative sign says move back, but the other negative sign says, move back backwards, so move forwards. Right, the answer then is 10000+10.9=10010.9.