Two canoes are tied together and traveling down a straight river with the current. Spotting a waterfall, the two canoeists cut the rope and push off of each other to try and reach opposite banks of the river. They are in the middle of the river and will reach the waterfall in 4 s. ONe canoe and its rider have a combined mass of 85 kg and reach the shore in 3.4 s. The toher canoe and rider reach the opposite shore just in time at 3.9 s. What is the combined mass of this canoe and rider?