You can solve any proportion using the fundamental rule of proportions. First you must build the proportion if you are given a word problem. Decide using logic whether it is a direct proportion (more on one side means more on the other). If so, find the parts of the equation that use the same units and make a fraction with these, putting the smaller number on top. The number left over is linked with one of the other two numbers in the fraction--use logic to figure out which. Put it in the same position as the number it is linked with, and put the unknown on the other side. Now you have two fractions on either side of an equals sign. You can use cross multiplication to isolate the unknown and solve for it. If you follow this video you will understand and be able to use this principle to solve for an unknown in a proportion.