In this video the instructor shows how to use proportions to solve fractions. When you have a proportions problem with an unknown term, cross multiply and divide it to get the value of that unknown term. For example if given 7/8 = m/4, cross multiply 7/8 with 4, giving 7/2 which is the value of the unknown variable m. So if two quantities are proportionate then you can equate them as shown in the video and cross multiply to get the value of any unknown variable. Proportions are just fractions which are equal to one another. The instructor goes on and solves an example to explain the concept.