The key to solving this problem is the fact that best glide speed is, in fact, best gliding AoA. Since the aerodynamic shape of the aircraft remains unchanged, so does the AoA.
Because L=0,5*rho*CL*IAS^2*
and rho, CL(AOA(bestglide)) and S being constant,
Lift is proportional to IAS squared, thus,
IAS is proportional to the square root of lift.
Given that the amout of lift needed is proportionate to the weight of the aircraft we can state that
(NewIAS/OldIAS)=(root(NewWeight)/root(OldWeight)), you get
NewIAS=OldIAS*root(NewWeight/OldWeight)
and with numbers that turns out to be
NewIAS=100*root(845/500)
NewIAS=130
as stated by the previous poster
