Paco is correct in saying O level standard is required. Simply a question of putting in time and effort to learn the "high class rubbish"...... ( quote from Sandy Thomson). Enjoying mental arithmetic and sums generally is important.
I would say that 75+% you can bin once on line.
The Met you learn you use daily. As for the rest....the odd bits.
Post ATPL studies there is a whole new raft of stuff to learn which deals with the practicalities of airline flying.
Try and study a bit daily. Nibble away at it.
