單位轉換的問題

問題應該是：
It has been estimated that there is 4 x 10-4 mg of gold per liter of seawater. At price of $19.40/g, How much(USD) of the gold in 3.1 km3 (1km3=1 x 1015 cm3) of the ocean?
As 1 mg = 10-3 g and 1 liter = 1000 cm3,

4 x 10-4 mg per liter
= 4 x 10-4 x 10-3 g / (1000 cm3)
= 4 x 10-10 g / cm3

Hence amount of the gold (in USD) in 3.1 km3 of the ocean
= (4 x 10-10 g / cm3) x ($19.40/g) x (3.1 km3) x (1 x 1015 cm3 / km3)
= $24056000

1) 1L = 1000cm^3, 1g = 1000mg
[4 x 10^(-4)] mg/L x (1L/1000cm^3) x (1g/1000mg) = 4 x 10^(-10)g/cm^3
2) 3.1km^3 x 1015cm^3/1km^3 = 3146.5cm^3
3) 4x10^(-10)g/cm^3 x 3146.5cm^3 x $19.4 = $24.42 x 10^(-6)