Answer
$v_B = 4.86m/s$
Work Step by Step
$E_B=E_A$
$K_B +U_B = K_A +U_A$
$v_A = 0$ which means $K_A = 0$
$K_B +U_B = 0 +U_A$
$K_B +U_B = U_A$
$K_B = U_A-U_B $
$\frac{1}{2}mv_B^2 = mgh_A-mgh_B $
$v_B^2 =2 \times \frac{mgh_A-mgh_B}{m} $
$v_B^2 = 2gh_A-2gh_B$
$v_B^2 = 2g(h_A-h_B)$
$v_B = \sqrt {2g(h_A-h_B)}$
$v_B = \sqrt {2\times 9.8m/s^2 (20m-(20\times cos20^{\circ}))}$
$v_B = 4.86m/s$
