An estuary has volume $V = 10^7$ m$^3$. Freshwater enters at a rate $Q = 10^5$ m$^3$/day, and tidal exchange adds a mixing flow rate of $T = 4 \\times 10^5$ m$^3$/day. The average residence time of a water molecule in the estuary is given by $\\tau = \\frac{V}{Q + \\eta T}$, where $\\eta = 0.5$ is the tidal exchange efficiency. What is the residence time $\\tau$ in days?