Ok so in One kilogram of seawater contains 5 × 10 ^-3µg (part per billion) of gold.
and 1 microgram = .000000001kg (5 x 10^-9)
**i need to find out how many kg of seawater I will need to have 1kg of gold?*

I first thought of seeing how many kg of gold I have in 1kg of seawater and got
5 x 10^-12
but I do not know where to go from there can anyone help thanks!

again ** I need to know how many kg of seawater I would need to get 1kg of gold**

It's simple proprotions;

For example, if 2 apples cost $0.10
Then 5 apples will cost $0.25

\frac{0.10}{2} \times 5

If 5 x 10^-12 kg of gold is present in 1 kg of water, then
1 kg of gold will be present in

\frac{1}{5\times10^{-12}} \times 1