Wt of solid in air = 7.5 gf
Wt of solid in water- wt of sinker in water=62.5gf
Weight of sinker in water=72.5 gf
Weight of solid in air+weight of sinker in water=7.5gf + 72.5gf= 80gf
Weight of solid in air- weight of solid in water= 80gf - 62.5gf
RD = 7.5/17.5 = 0.428