What is the fourth proportional to 0.6, 0.12, 0.04?

Let the fourth proportional be x.
By the definition of proportion:

0.6:0.12::0.04:x
0.12
0.6
​
=
x
0.04
​

Simplifying the left-hand fraction:

0.12
0.6
​
=
12
60
​
=5
Now, substitute back into the equation:

5=
x
0.04
​

x=
5
0.04
​
=0.008