W=20 e(-kt)  
A. Rearranging gives k= -(ln(w/20)/t 
 Substituting w= 10 and solving gives k=0.014 
 B. Using W=20e(-kt). After 0 hours, W=20. After 24 hours, W=14.29g. After 1 week (24x7=168h) W=1.9g 
 C. Rearranging gives t=-(ln(10/20)/k. Substituting w=1 and solving gives t=214 hours. 
 D. Differentiating gives dW/ dt = -20ke(-kt). Solving for t=100 gives dW/dt = 0.07g/h. Solving for t=1000 gives 0.0000002g/h 
 E. dW/dt = -20ke(-kt). But W=20e(-kt) so dW/dt = -kW
        
             
        
        
        
The primary force would be Thermal Convection, it pushes it and thus makes continents drift.
-Hope that helps, Nexxmexx :3