We were doing a room and had a situation where we had to try and get a door open. The door was magically sealed and required a DC 30 Thievery or Strength check. We were level 7, and my character was the only one who could open the door (being a rogue with 15 thievery). We have two people with a Strength of 20,21. We assumed it meant the Strength modifer for the check plus 1/2 level. But that way neither of them would be able to open it, since their highest roll would be a 28. How does a Strength check like that work? Is there a way to train in Strength?