Theophylline is a pharmaceutical drug that is sometimes used to help with lung function. You observe a case where the initial lab results indicate that the concentration of theophylline in a patient's body decreased from 2.0 x 10⁻³ M to 1.0 x 10⁻³ M in 24 hours. In another 12 hours, the drug concentration was found to be 5.0 x 10⁻⁴ M. What is the value of the rate constant for the metabolism of this drug in the body?