Answer :

Solution :

As acceleration of point Q is zero, so this rigion body PQR is hinged at Q.
                                    a_RP = a_R — a_P
is given 10 m/s* an angle of 180°, that means only radial acceleration is hence at
that instant and reference is PR
a_RP = (RP)ω² = 10

20ω² = 10

      ω = 1/√2

as α of whole body remains same so point R has only radial acceleration at that instant

a_R  =  QRω²  = 16*1/2 = 8m/s²

and will be in the horizontal backward design, but our reference is only PR.

So the angle of it from reference is (180 + Θ) .

from ∆PQR         tanΘ = 12/16 

                                    Θ = 36.87

so, 180+ 36.87 = 216.87

So answer is 82217°