Step-by-step explanation:
[tex]scale \: factor = \frac{7}{28} = \frac{1}{4} \\ [/tex]
[tex] ratio \:of \:surface \:areas\\
=\frac{2[5\times 7 + 7\times 3+3\times 5]}{2[20\times 28+ 28\times 12+12\times 20] }\\\\
=\frac{[35 + 21+15]}{[560+ 336+240] }\\\\
=\frac{71}{1136}\\\\
=\frac{1}{16}\\\\
= 1\: :\: 16[/tex]