The function is given by
[tex]\\ \tt\hookrightarrow D(p)=1500-25p[/tex]
[tex]\\ \tt\hookrightarrow 1500-25p\geqslant 0[/tex]
[tex]\\ \tt\hookrightarrow 1500\geqslant 25p[/tex]
[tex]\\ \tt\hookrightarrow p\leqslant 60[/tex]
Now
[tex]\\ \tt\hookrightarrow D_f\in (-\infty,60][/tex]