Answer:
(a) 525 g
Since they're proportional to each other:
[tex]\frac{20cm^{3} }{210g} =\frac{50cm^{3} }{Xg}[/tex]
Cross-multiply & solve:
[tex]20x=210*50\\20x=10500\\x=\frac{10500}{20} =525[/tex]
(b) 38 cm^3
Same thing, different numbers:
[tex]\frac{20cm^{3} }{210g} =\frac{Xcm^{3} }{399g}[/tex]
Cross-multiply & solve:
[tex]210x=20*399\\210x=7980\\x=\frac{7980}{210} =38[/tex]