Excel Solver Optimize Non-Linear Problem

I attached a simple example of the problem I'm facing. I'd like to maximize a cell that is the sum of other cells that are not linear. In the spreadsheet, I have Z scores for categories I and II (I hardcoded in a mean of 30 and stdev of 15 for category I and 40 and 10 for category II), but do not want to maximize those z-score sums. I want to maximize the sum of their normdist values in G13. Clearly, solver won't find a linear solution. I have heard there was a way to get around this by doing piecewise-linear approximation. This is not a project that will take long to complete. You may try a few different methods to get this to work, but for the most part you should have a solution in your head before starting. Please let me know if you've found a solution and then if it works, I will hire you and complete the transaction.