It is estimated that uranium is relatively common in the earth’s crust, occurring in amounts of 4 g/metric ton. A metric ton is 1000 kg. At this concentration, what mass of uranium is present in 1.0 mg of the earth’s crust?

Answer 1

Use ratio/proportion as follows:
(4 g/1 x 10^6 g) = (x g/1 x 10^-3 g)
solve for x.
Or set it up by dimensional analysis.
1 x 10^-3(crust) x (4 g U/1 x 10^6 g C) = ??
Check my arithmetic. It looks like 4 x 10^-9 g or 4 ng/metric ton.