11.   To determine economic viability, developers calculate the ratio of usable, meaning sellable or rentable, square feet to overall square feet.

12.   But he urged the government to clarify how to calculate the median rent-to-income ratio.

13.   By calculating the ratio of uranium to lead, they determined that a dozen of the grains were very old indeed.

14.   Researchers calculated the ratio of available men to available women, because each spouse would face competition from unmarried members of his or her own sex.

15.   Then they calculated the ratio of these two figures.