Say a normal day goes like so: the cab leaves CERN @ 10 am, arrives @ Toby's @ 12 noon and gets back to CERN @ 2 pm.

On the day Toby walks, he leaves @ 11 am (1 hour early).

The 20-minute early arrival is due to not needing to drive the "dashed" part of the above. Each "leg" of this, on a normal day, takes the same amount of time, which therefore must equal 10 minutes.

So, the cab must reach the point where they meet @ 11:50 am, which is 50 minutes after the time Toby left.