The three masses shown in the accompanying diagram are equal. The pulleys are small, the string is lightweight, and friction is negligible. Assuming the system is in equilibrium, what is the ratio $a/b$? The figure is not drawn to scale!
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(A) $1/2$
(B) $1$
(C) $\sqrt{3}$
(D) $2$
(E) $2\sqrt{3}$
1. **Determine the tension in the string:**
   Since the system is in equilibrium and the masses are equal, the tension \( T \) in the string must balance the weight of each mass \( mg \). Therefore, the tension \( T \) in the string is:
   \[
   T = mg
   \]

2. **Analyze the geometry of the system:**
   The two endpoints of the string are separated by a horizontal distance \( a \). The middle mass is suspended such that it creates two equal segments of the string, each making an angle with the horizontal.

3. **Determine the angle of the string:**
   Since the middle mass is equidistant from both endpoints, the string forms an isosceles triangle with the horizontal distance \( a \) and the vertical distance \( b \). The angle \( \theta \) that the string makes with the horizontal can be found using trigonometry. Specifically, the tangent of the angle \( \theta \) is given by:
   \[
   \tan \theta = \frac{b}{a/2} = \frac{2b}{a}
   \]

4. **Use the known angle for equilibrium:**
   From the problem, we know that the angle \( \theta \) is \( 30^\circ \). Therefore:
   \[
   \tan 30^\circ = \frac{\sqrt{3}}{3}
   \]

5. **Set up the equation and solve for the ratio \( \frac{a}{b} \):**
   Equating the two expressions for \( \tan \theta \):
   \[
   \frac{\sqrt{3}}{3} = \frac{2b}{a}
   \]
   Solving for \( \frac{a}{b} \):
   \[
   \frac{a}{b} = 2\sqrt{3}
   \]

The final answer is \( \boxed{2\sqrt{3}} \).