The key to solving this type of problem is to realise that the acceleration of B is half the sum of the accelerations of A and C. Note also that there is only one string passing over free moving pulleys so the tension in the string is the same at all points.

1. Fill in the required masses. Press Apply
2. Press Release Mass
3. If you want to see the correct accelerations press Answers
4. Calculate the accelerations yourself and compare with the above values
5. Press Reset to repeat the simulation with different masses

   Note: The positive direction of the accelerations of A and C is chozen as down, of B is up. If the actual directions are opposite, a negative answer is given.