we can use the formula
[tex]F=m\times a[/tex]where F is the force, m the mass and a the acceleration
now, replacing
[tex]400=m\times21[/tex]then solve for m
[tex]\begin{gathered} m=\frac{400}{21} \\ \\ m=19.05 \end{gathered}[/tex]the mass is 19.05Kg
