Mental math is always used for estimations, not accuracy! I think one of the problems today is that people want to try to get an answer correct to nearest decimal, one must accept that this kind of accuracy is not required in flight! Calculations for hold corrections, wind corrections, time corrections etc. Will be sufficient if estimated.
Any pilot that can't mentally estimate an approx time knowing example your distance to go is 120 miles and you cruising at 210 kts, should not even be in a cockpit. Multi tasking is probably more the problem, to do mental calculation when you busy flying the aircraft.
From what I have seen maths within aviation is pretty basic, but try to get an easy math question first time you doing an ndb approach.