That's a common misconception. You can't make the calculation purely on the mass of raw material needed to get a X kg of Y% material.
Every enrichment process drops in efficiency as the enrichment increases, asymptotically approaching that process's enrichment ceiling.
That's why nobody uses thermal diffusion to enrich uranium: It's by far the easiest method (you can do it with 1800's technology), but the practical maximum enrichment fraction is too low for anything useful (other than providing feedstock to better enrichment processes).
That's also the reason a centrifuge enrichment plant uses warehouses full of cascaded centrifuges: The centripetal force inside the centrifuge is working against a diffusion gradient, so each stage only gives a small percentage decrease of the "derichment" fraction.
Once you pass 50%, you stop looking for U-235 needles in a haystack to keep and start looking for U-238 needles in a haystack to throw away.
Every enrichment process drops in efficiency as the enrichment increases, asymptotically approaching that process's enrichment ceiling.
That's why nobody uses thermal diffusion to enrich uranium: It's by far the easiest method (you can do it with 1800's technology), but the practical maximum enrichment fraction is too low for anything useful (other than providing feedstock to better enrichment processes).
That's also the reason a centrifuge enrichment plant uses warehouses full of cascaded centrifuges: The centripetal force inside the centrifuge is working against a diffusion gradient, so each stage only gives a small percentage decrease of the "derichment" fraction.
Once you pass 50%, you stop looking for U-235 needles in a haystack to keep and start looking for U-238 needles in a haystack to throw away.