In the program there is this constraint
a set of activities occupies max time slots from selection
I suggest adding a new constraint :
a set of activities occupies min time slots from selection
You can use the negative of the official constraint.
How do I use
40 slots per week, 20 activities. You want in 15 slots min 5 activities. Add constraint for the other 25 slots, max 15 activities.
Thank you very much