C. Cello playing music at a concert
        
                    
             
        
        
        
density = mass /volume
1)mass of rock = 26.0 gram
2)volume of rock = volume of water it has displaced= 25.3-13.12 = 12.18 
put the value of mass and volume in first equation and get the density value 
 
        
             
        
        
        
1/5 + 1/4 =1/x
4x + 5x = 20
9x = 20 
x = 2.22 hours  Answer
        
             
        
        
        
Average atomic mass is the weighted average atomic masses with regard to the relative abundance of the isotopes 
average atomic mass of Li = relative abundance of Li-6 x mass of Li-6 + relative abundance of Li-7 x mass of Li-7
average atomic mass of Li = (7.42% x 6.0151 a.m.u) + (92.58% x 7.0160 a.m.u)
                                           = 0.446 + 6.495
                                           = 6.941 amu
average atomic mass of Li is 6.941 amu
        
                    
             
        
        
        
Based on Le Chatelier's principle, if the equilibrium of a system is disturbed by changing the temperature, pressure or concentration, then it will shift in a direction to undo the effect of the induced change.
The given equilibrium is:
A + B ↔ AB
Removal of the reactant A implies that the concentration of A has decreased, therefore the equilibrium will shift in a direction to produce more of A. Thus, it will shift to the left and the rate of the reverse or backward reaction will increase.