I would say that you need to be fairly fast in basic calculation. F.ex. during training you will need to calculate required rate of descent on an approach. Lets say you have a groundspeed of 120 kts, which is the same as 2 NM
pr minute. When you are 10 NM from the runway you start your descent from 3000 ft, to cross distance 5 not lower than 1500 ft (minimum altitude at this point) . Your rate of descent will be: 5 NM divided by 2 NM
pr min = 2,5. 3000 ft - 1500 ft = 1500 ft. 1500 ft divided by 2,5 is 600. So you need to descend from distance 10 with a rate of 600 ft per minute to pass 5 at 1500 ft. I know this was very specific, but this is the kind of stuff you will have to be able to do. On larger aircrafts I would say numerical reasoning, crosschecking the information provided on the displays, will be more important...and of course the 3 times table he he, already mentioned.