Someone came into a store and stole 100 dollars from the register. the person returned 5 minutes later and purchased 70 dollars in merchandise with the same 100 dollars he stole i gave him 30 dollars change. how much money did the store loose.

Think of it this way. Ignore for now that $100 was stolen.

The purchase of the $70 item for $100 cash with $30 change is a perfectly fair purchase. The store received $100 cash, and the store gave $70 worth of merchandise plus $30 cash.There was no loss to the store there.

The fact that $100 in cash was stolen earlier from cash register means the loss is $100. The legitimate transaction does not affect the loss.

If you have a hard time understanding the loss is $100, then think of it this way.
Reverse the order of the two happenings.

A person walked into a store and bought a $70 item with a $100 bill. He received $30 change. So far, there is no loss to the store. Everything is legit.

That customer later came back to the store and stole $100 from the cash register.

Here we see clearly that the loss is exactly $100. It is simply the $100 stolen from the cash register.