You have been asked to measure the volume of a small lake. You dump in 5.0 L of a 2.0-mol/L solution of a dye, which degrades, with a half-life of 3.0 days. You wait exactly one week for the lake to become well mixed (during this time, assume no water is lost); you then take a 100-mL sample. The dye's concentration in this sample is 2.9 x 10"6 mol/L. What is the lake's volume?