Physics Pulley

Four light frictionless pulleys and a single piece of strong light rope are arranged to connect the three tanks. Each straight section of rope was either vertical or parallel to the frictionless 30 degrees slope. The masses of tank A and B are 90kg and 30kg respectively, and tank A had an acceleration of 3.0m/s^2 downward when all three were allowed to move. Determine the unknown mass of tank C (which was on the 30 degrees slope).