to beat 21, u have to enter 5 different portals
it's hard to describe but you enter the one with the medium line on the upper right side
then the one with the big line directly to your left
then the one one down from it, with the small line
then you enter the one with the small line, above the one you just entered
then the one with the big line directly above you
every line you enter a correct one, marks appear in the upper right hand corner