Proportions or ratios are fundamental concepts of mathematics. A proportions is an equation that states that two ratios are equal. Hence proportion can be written in two ways as a:b=c:d or a/b=c/d. In these equations a and d are called as extremes and b,c are called as means. So when working with proportions we can state that product of the means is equal to the product of the extremes i.e. a*d = b*c. This is obtained by cross multiplying the second form of proportions. In mathematics you are frequently faced with proportion problems where one number is missing in a proportion. To solve such problems cross multiply the numbers to obtain the result. This video teaches proportions and solves some problems involving proportions.
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