Because it is a diamond (note: at this time, the diamond should be used as a known condition; To solve), so PB=PC, BP=7-3t,

And PC∨bo, that is, △APC∽△ABO, deduced AP:AB=PC:BO,∴(3t-4):3=(7-3t):5, and got t=4 1/24.

Then PC=45/24, PA=27/24, in Rt△APC, Pythagoras PC=36/24.

Then AC-t=4 1/24-36/24=5/24. So we should leave 5/24 seconds late.