Side 1 6 fps
2 6 fps
3 6 fps
4 5 fps
Let x = distance of each side (it will cancel out)
x ft / 6 ft/sec x 3 + x ft / 5ft/sec = 4x / (avg)
(3/6 sec + 1/5 sec ) x = 4x / avg Divide both sides by 'x'
15/30 + 6/30 = 4/avg Solve for avg (which is average speed in ft/sec)
avg = 4 / (21/30) = 120/21 ft/sec = 40/7 ft/sec