if u could get 90 in 3unit, then ur 2unit maths would be mid 90s anyway
if u did 4unit, and neglected the ext. 2, did only ext. 1
u would need to do 2 extra units then, and if you get 90 in that ext 1 maths, you would've done better if u had done 3units anyway...
hope im not too confusing