Let's do what the questioner thinks.

Party A completes 1/20, Party B completes 1/30, and Party C completes 1/40 per hour.

So in 12 hours, B completed 12/30, C completed 12/40,12/30-12/40 = 0.3.

So A only worked for six hours, 12-6=6, which means A paused for six hours.