Okay...
Some ideas to lead this into a direction. Following your example: (10+20+30) / 3 = 16.25
You are searching for a fourth value to add that leads to a mean of 18.125. So: 18.125 * 4 = 72.5
Minus the three given values: 72.5 - 60 = 12.5
This is 5 * 2.5
The next question is: Can this be taken as a general rule? Or is this factor (2.5) for the next value to add depending on some more?