Two immiscible fluids are contained between infinite parallel plates. The plates are separated by distance \(2 h\), and the two fluid layers are of equal thickness \(h\); the dynamic viscosity of the upper fluid is three times that of the lower fluid. If the lower plate is stationary and the upper plate moves at constant speed \(U=20 \mathrm{ft} / \mathrm{s}\), what is the velocity at the interface? Assume laminar flows, and that the pressure gradient in the direction of flow is zero.