Determine the force in a pulley system

[Zheng_]

Homework Statement

If A has a mass of 300kg, determine the mass of B if the system in figure P4-28 is to be in balance.
(What I couldn't understand is the 2T_b in the answer.)

Homework Equations

ΣF_y=0[/B]

The Attempt at a Solution

Since the pulley on top is connected with two pulley at the bottom, its force is doubled?

 

[gneill]

The middle pulley (above block B) has two tensions T_b pulling it downward (red string in figure below). So what must be the tension in the blue string?