Years ago when I was in the old CAA, I was checking out a new (young) guy for a rating on a certain sector. One aircraft this guy was responsible for was tracking between two reporting points that were 119 nm apart and the pilot's ETI for the route segment was 60 mins. Casually I ask the guy I was checking what was the pilot's approximate groundspeed.
To my surprise he reaches for a calculator! I still shake my head at that one.
Surely there must still be a need for some mental maths ability!