How to calculate the ratio of 18o/16o in a meteorite?

Question

a grain has a 18o/16o ratio of 9.52x10^-3 and its 17o / 16o ratio is 3.28x10^-4

calculate the delta 18 o and delta 17 o of the grains relative to the earths oceans,

use a value of 1/499 for ratio of 18 o / 16 o and a value of 1/2690 for the ratio of 18 o / 17 o in the earths oceans

the answer i get is very very high and doesnt seem to make sense

Answer 1

Oxygen 18 appears to be about 5 times more common in the meteorite and Oxygen 17 0.9 times.

This makes sense to me.