A proportion is a pair of equal ratios. We can write in Fractional form, which is like:
4 =  8
5    10  as 4:5= 8:10, where 5 and 8 are the means while 4 and 10 are the extremes. If the product of the means and extremes are equal, we can say that the pair of ratios form proportion. Each part of proportion is called a term