This is basically a conversion factor problem...
Start with the given value, 1.42 ct, and then apply the conversion factors below to transform into a volume measured in cubic-centimeters (cm3)
1.42 ct · (200 mg / 1 ct) · (1 g / 1,000 mg) · (1 cm3 / 3.51g) = 0.08091
=> 0.0809 cm3 = 8.09 × 10-2 cm3
(rounded to correct number of significant-figures)

John H.
09/02/19