Mass  m = 85 kg
diameter of the stool d = 2.0 cm
= 0.020 m
  Y = F /A ( L0/ΔL )
       ΔL /L0 = F
/ A Y
                    =
( w / 3 ) / pi r^2 Y = mg / 3pi( d/2 )^2 Y
                                                  =
4 mg / 3pi d^2 Y
          Y is
the young's modulus Y =1*10^10 N /m2
= (4 * 85 * 9.81) / (3 pi * 0.020^2 * 1 * 10^10)
                              =
8.847 *10^-5 m
  percentage change = 0.0088 %
(Answer)
I hope help you.....!!
