To clarify the situation, imagine that you sign up for training on a motorcycle track. Training takes place on Fridays and Saturdays. There are two groups: beginner and advanced. You can buy a ticket for Friday, Saturday or both. Both on Friday and Saturday we have 15 places in each group.
When selecting, you must select a group (beginner / advanced). I would settle the choice between Friday and Saturday and groups with variants, but what if someone buys a ticket for both days and goes, for example, to a beginner group. The system then has to subtract one place in groups beginning on Friday and Saturday.
I planned it as a function that, when choosing from the option days option for both days and then selecting a group, will subtract 1 place from the same group on Friday and the same group on Saturday.
