You need to start by considering what forces are on the 20kg block. The 20kg block will have a tension force as well as a gravitational force that are antiparallel to each other. There is also a normal force, but we will not need to consider this since friction is negligible. The equation will be: [tex]m_1a_1 = T - m_1g\cos\phi[/tex], where [tex]m_1 = 20kg[/tex]. From this equation, you can find the expression for acceleration ([tex]a_1[/tex]) by dividing by the mass.
