Figure shows two sections of an old pipe system that runs through a hill, with distances dA

Question:

Figure shows two sections of an old pipe system that runs through a hill, with distances dA = dB = 30 m and D = 110 m. On each side of the hill, the pipe radius is 2.00 cm. However, the radius of the pipe inside the hill is no longer known. To determine it, hydraulic engineers first establish that water flows through the left and right sections at 2.50 m/s. Then they release a dye in the water at point A and find that it takes 88.8s to reach point B. What is the average radius of the pipe within the hill?