**1. The problem statement, all variables and given/known data**

No friction, pulleys and also string are weightless, the tension through the string is distributed uniformly. Find the acceleration of the mass 3M (see image attached)

**2. Relevant equations**

II law of Newton

**3. The attempt at a solution**

Well I wrote such equations for all pulleys (counting pulleys from right to left):

1) M*a1 = T – Mg;

2) M*a2 = 2T – Mg;

4) M*a3 = 2T – Mg;

5) 3M*a = T – 3Mg.

Now we have 1 more unknowns than equations. Any ideas?

http://ift.tt/1ol92ks