Answer:
If a:b :: c:d or in other words a:b = c: d, then the quantity 'd' is what we call the fourth proportional to a, b and c. For example, if we have 2, 3 and 4, 5 are in the proportion such that 2 and 5 are the extremes, then 5 is the fourth proportional to 2, 3, and 4.