Let the height of the desk be h m. Time of fall of the eraser from the top of the desk being t=0,77s we can write by considering the motion under gravity
h=0xt+1/2gt2
h=1/2x9.80x0.772=2.91 m
If the height of the school building be H m and angle of elevation of sun be A then
h/s=H/S=tanA , where s and S are length of shadows of desks and hall.
given s =2.92 m. S =8h=8*2.91m
now h/H=s/S=2.92/(8×2.91)
So H = 8h = 8×2.91m=23.28 m