Comment by keyraycheck

That is true for deep work like:

- solving mathematical problems

- coding non-trivial stuff

- teaching non-trival stuff

The challange is shallow work e.g.

- replying to emails

- meetings

- scheduling meetings

And than there is no-work:

- checking your phone

- lunch

- walking and chatting before the next meeting

Shallow and no-work can easily eat anywhere between 20 and infinity :)