Limit order matching - same time

Given these two orders with the same size:

Buy  x for 20
Sell x for 15

What price are they matched/should they be matched if we have no time priority? Are there any market standards to solve this case?

I read that in most systems there is price/time priority but in this case the assumption is, there is no way to figure out which order was first because we are on a blockchain and in this case, both have the same time stamp.