What is the meaning of 后 in ~分钟后?
二十分钟后 = "20 minutes later" or "after 20 minutes" 

Example: 

二十分钟后就到了 (will arrive 20 minutes later/ will arrive after 20 minutes) 

二十分钟前 = "20 minutes earlier" or " 20 minutes before"

二十分钟內 = "in 20 minutes"