Question

Zack and tia played chess for 50 minutes . they put the chessboard away at 11:20 which clock shows the time they started their chess game

Answer 1

. . . Well, I don't know what clocks you are referring to.
But if they played for 50 minutes, and stopped playing at 11:20 PM, rewind 50 minutes before that, and they will have started playing at 10:30 PM.

But if they stopped playing at 11:20 AM, rewind 50 minutes before that, and they will have started playing at 10:30 AM.