Question 38982
using the equation d1(w1) = d2(w2)

Joe = 40 lbs(8)
Jane = 70 lbs(5)
Bill = 90 lbs(3)
Father = 188 lbs(x)

Therefore
40(8) + 70(5) + 90(3) = 188(x)
320 + 350 + 270 = 188x
940 = 188x  divide both side by 188
x = 5

Joe, Jane and Bill's father must seat 5ft from the fulcrum.