16.
Explanation
The order of convergence of the secant method is approximately 1.618. This value is often referred to as the "Golden Ratio" and is an interesting feature of the secant method. The method's order of convergence means that, on average, the number of correct decimal places roughly increases by 1.618 with each iteration, which is faster than linear convergence (order 1) but slower than quadratic convergence (order 2) seen in some other methods like Newton's method.