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