1. The problem statement, all variables and given/known data The problem is as shown in the attatchment. 2. Relevant equations The relevant equations are also given in the attatchment. 3. The attempt at a solution My problem is how to adapt the given formula in order to find the sum of the function k(40-r) Do i use the formula for 1 to 40, then 1 to 20 and subtract? Even just a hint in the right direction would be useful. This sorta stuff has never been my strong point.