0.105 is 0.05 times a number.
Let x represent the unknown number.
Which pair of equations are both true?
     0.105x=0.050.105x=0.05 and  x=0.00525x=0.00525 
    0.05x=0.1050.05x=0.105  and  x=2.1x=2.1 
     0.05x=0.1050.05x=0.105  and  x=0.00525x=0.00525  
    0.105x=0.050.105x=0.05 and  x=2.1x=2.1