Long long time since I have thought of this but sees to recall it's a square root relationship - if you double the weight, the stall speed increase by a factor of the square root of 2 = (about) 1.4 so in this case the new stall speed is 140 kts.
No doubt those more educated can explain this!
A good book, although quite old, is "Mechanics of Flight" by A.C. Kermode which explains much of this stuff!