a.
Wa = Wđa + Wta = mgzA
=> Wa = 1.10.20 = 200 (J)
Wa = Wb
<=> 200 = Wđb + Wtb = $\dfrac{1}{2}$m$Vb^2$
<=> 200 = $\dfrac{1}{2}$.$Vb^2$
=> Vb = 20 m/s
b.
Wđb - Wđa = Ap + Afc
<=> $\dfrac{1}{2}$m$Vb^2$ - $\dfrac{1}{2}$m$Va^2$ = mgh - Fc.h
<=> $\dfrac{1}{2}$$Vb^2$ = 10.20 - 5%.10.20
=> Vb ≈ 19,49 m/s