This is the third Riddler trial and can only be accessed after passing the second.
Upon returning to the orphanage after passing the second riddler trial, the numeracy 101 door will be unlocked.
Send bats to blue, cats to pink pressure plates,
Control catwoman and look at the blue number sequence on the screen.
Remember it, change to bats then use your batarang on the question marks indicated by the pattern.
1 is the most left ?, 5 the most right.
Once done, control bats and read the pink number sequence. Remember it, switch to cats and enter the number in order again.
The gates will drop, beat the goons and collect your key and your achievement. Really simple puzzle that should pose no problems.