Need to build 30 desks with demensions 70cm, 30 cm and 20 c how much wood do I need

Is this a math problem or are you being serious? In real life you would have to account for mistakes, how exactly the parts would be joined (would there be re-enforcing pieces on the corners) etc and also your not mentionning how wide the boards are.

If this is a math problem, it's pretty simple multiplication as it sounds like a real life situation dumbed down into a math problem.

If this is real, just buy a lot! Go somewhere you can return what you don't use.

Post a pic of what one should look like, also covert the cmeters to inches

yes, I work with inches and feet, not cm. are there to be drawers, slide outs, or shelves?? Too many variables to figure with just the info given.

Sounds like a homework question. There is not way for us to answer this question based on those 3 dimensions.