Method 1 is to build it from the bottom and how much you should spend on things. This way regularly broke down for me because I could not account for things that came up. These would be things like medical expenses or other things such as heading out with friends etc.
Let m be the weight of math textbook. Let x be the weight of language arts class. Math textbook weighs 4/5 of a pound less than 4 times the weight of the book <span>for language arts class: m = 4x - 4/5