Cost Minimization assuming hourly paid employees

I wish I knew a way to get the minimized solution using Maple as a
function of maximum time allowed to complete the projects.

The problem does provide the following insights:
>If t<9.94 then no solution exists
>If 9.94<t<18, then maximum allowed time to complete the project should
be used to end up with the minimum cost.
>If t>18, at most 18 units of time should be used to complete the
project