Explanation: Ratios can be written as fractions, so start with the ratio of the densities of aluminum and lead. Find the numbers you need from the chart, 1.71 and 11.4 respectively, and write them as a fraction set equal to the given ratio of 3 over x, like this: 11.41.71​=x3​Cross multiply to get 1.71x=34.2, then divide both sides by 1.71 to get x=20. Now use this value in the second ratio: 9.0 osmium ​=820​Cross multiply to get 8( osmium )=180, then divide both sides by 8 to get osmium =22.5.