Multi-level aggregation using DAX

Hello,

I've encountered a problem when writing a DAX measure.

My design:
- A fact table

- A Date dimension (granularity: day)

- A Time dimension (it has Hour integers and Minute integers - only for quarters (0, 15, 30, 45)

What I want to achieve:

- Force Tabular to always AVERAGE my measure from 15-minute intervals to an hour.

- Force Tabular to always SUM the averages from first bullet.

- This has to work even when Time/Date dimensions aren't used in the pivot table/PBI.

Let's suppose this is how data at the lowest granularity looks like (there are more dimensions, though):

This is how it should look like one level higher - no more minutes:

This is how it should look like if we remove Time dimension:

And if I remove the Date dimension:

And If I don't use any dimensions:

Can anyone help me?How to achieve this in DAX?

Thanks in advance.