As per Newton's II law we know that
[tex]F = ma[/tex]
here we know that
[tex]F = 20 N[/tex]
[tex]a = 1.5 m/s^2[/tex]
now the mass of the object will be given as
[tex]m = \frac{F}{a}[/tex]
[tex]m = \frac{20}{1.5}[/tex]
[tex]m = 13.3 kg[/tex]
so mass of the object will be 13.3 kg