Sorry if this a stupid question, I am sure the term "A075" is very familiar to any experienced pilot, but I just couldn't find the explaination anywhere.

For questions regarding descent distance:

"You wish to plan your descent for an overhead arrival at 1500 ft AGL, at what distance to the nearest nm, should your descent start?
Destination elevation: 1000 ft
Cruise level: A075
Rate of descent: 600 ft/min
GS: 120 kt
Area and local QNH 1020"

My first questions is: A075 means the altimeter reads 7500 feet when setting subscale to area QNH 1020, correct?
The first step will be working out the verticle distance, since area and local QNH are the same, it is simply 7500 - (1000+1500) = 5000

My second questions is: What if last line of the question is "Area QNH 1015, local QNH 1020"?
We need to work out the altimeter reading when cruising, in local QNH, that will be 7500 + 5 * 30 = 7650
Then work out the verticle distance, 7650 - (1000+1500) = 5150
Is this correct?

Sorry if this a stupid question

There are no stupid questions. There are, however, silly people who are reticent to ask a question if they need an answer .....

What does cruise level A075 mean?

AXXX is a generic abbreviation for "altitude XXX". So A075 means altitude 7500 ft. See, for example, AIP ENR 1.10-23, Item 15.

The rest sounds OK to me.