The fourth proportional of 64, 80 and 88 is:

Let the fourth proportional to 64, 80, and 88 be x.
By the definition of proportion:

64:80::88:x
80
64
​
=
x
88
​

Solving for x:

x=
64
88×80
​

x=
4
88×5
​

x=22×5=110