If 80 is the fourth proportional to 10, 16 and x, then the value of x is:

If 80 is the fourth proportional to 10, 16, and x, then by the definition of proportion:
16
10
​
=
80
x
​

Now, solving for x:

x=
16
10×80
​

x=10×5
x=50
Thus, the value of x is 50.