A $3-$phase, $2-$pole, $50$ Hz$,$ synchronous generator has a rating of $250$ MVA, $0\mathrm{.}8$ pf lagging. The kinetic energy of the machine at synchronous speed is $100$ MJ$.$ The machine is running steadily at synchronous speed and delivering $60$ MW power at a power angle of $10$ electrical degrees. If the load is suddenly removed, assuming the acceleration is constant for $10$ cycles, the value of the power angle after $5$ cycles is $\_\_\_\_\_\_\_\_\_$ electrical degrees.