Mechanical advantage (MA) is calculated using the formula:
\[ \text{MA} = \frac{\text{Output Force}}{\text{Input Force}} \]
In this case, the output force (the weight of the rock) is 60 newtons, and the input force (the force you exert) is 15 newtons. Plugging these values into the formula gives us:
\[ \text{MA} = \frac{60 \text{ N}}{15 \text{ N}} = 4 \]
Therefore, the mechanical advantage is 4.