We know that for every number, to find its difference between that number and its opposite, is to double the number. For example, 10, the opposite of it is -10 and to find 10-(-10) mentally, we multiply 10 by 2=20. So easy! If you are asked about a number that is 20 greater than its opposite, then divide 50 by 2 and you are done!
25-(-25) =50, True.