Consider the reaction A + B → C. A series of reactions have been performed using different initial reactant concentrations, and the initial reaction rates have been measured:

1. [A] = 5 M; [B] = 5 M; Rate = 0.4 M s^-1
2. [A] = 2.5 M; [B] = 5 M; Rate = 0.1 M s^-1
3. [A] = 2.5 M; [B] = 2.5 M; Rate = 0.05 M s^-1

What is the value of the rate constant k?