Solver doesn't find global Max

I think Solver is having trouble with the integer constraint in B2. If I drop that constraint (=C2 instead of =INT(C2)), then my version of Solver seems to have little trouble getting really close from a variety of starting values. It's not perfect, but it gets really close.

If it is not possible to use the Solver, in my case, is it possible to use a VB script to test all combinations?

Does it have to be a VB script? You can do this using Excel's built in Data Table utility https://www.excel-easy.com/examples/data-tables.html [Enter =P8 in I24. Enter desired percentages in I25:I30. Enter Days in J24:P24. Select range and insert Data table using C2 for the row input cell and B3 for the column input cell (or vice-versa if I got them mixed up).] In fact, that was one of the first things I did to study your problem was to build a quick data table and chart it on a scatter chart so I could see what your objective function was doing at different input values. From this exercise, I could see that the maximum was always going to be at day 7, and would be near 90%.

Here's how I might approach this problem. Solver should have no problem finding the percentage in B3, once the best choice for days in B2 in discovered (I'm assuming there will be variations of the problem that won't maximize at 7 days, though this specific example appears to fix on 7 days). I might set up a data table like I've described, because that tool will allow me to quickly see that 7 (or whatever) days is the optimum choice for number of days. Enter that value in B2.

Then run Solver and tell solver to find the maximum of P8 by changing only B3. Solver should have no trouble finding that maximum.

If you feel that you must have Solver find both days and percentage in one go, then I would drop the integer constraint on B2 and let Solver find the optimum at fractional days.

Thoughts?

I have dabbled a little with open solver, but have not been sold on it. Other users here (@Alf) seem to have a lot more experience with it. I have never heard of What'sBest.