What is the fourth proportional of 21,56 and 27?

Let the fourth proportional of a,b, and c be d. By definition:
a:b::c:d⟹
b
a
​
=
d
c
​
⟹d=
a
b×c
​

Given values: a=21,b=56,c=27.
Substituting these values:

d=
21
56×27
​

Simplifying the fraction:

d=
3
8×27
​
(dividing 56 and 21 by 7)
d=8×9=72