Final answer:
To solve for a missing number in a ratio, set up a proportion and cross-multiply. To check if two ratios are proportional, cross-multiply and compare the cross-products.
Step-by-step explanation:
To solve for a missing number in a ratio, you can set up a proportion and cross-multiply to find the value. For example, if you have the ratio 2:5 and you know one of the numbers is 10, you can set up the proportion 2/5 = 10/x and solve for x.
To check if two ratios are proportional, you can cross-multiply and see if the cross-products are equal. If the cross-products are equal, then the ratios are proportional. For example, if you have the ratios 2:5 and 4:10, you can cross-multiply and see that 2 * 10 is equal to 5 * 4, showing that the ratios are proportional.