text: solutions to equation!? how many solutions are there to the equation x1 + x2+ x3 + x4 = 13  where  x1,  x2, x3, x4 are non-negative integers?  full working needed. guess x1>=x2>=x3>=x4. work systematically: 13 = 13 + 0 + 0 + 0 = 12 + 1 + 0 + 0 = 11 + 2 + 0 + 0 = 11 + 1 + 1 + 0 = 10 + 3 + 0 + 0 = 10 + 2 + 1 + 0 = 10 + 1 + 1 + 1 =  9  + 4 + 0 + 0 = go on yourself and count the number of possibilities at the and. good luck  th