Take the arithmetic mean, or the average of the two values to get the halfway point.
[tex]\dfrac{4+16}{2}[/tex]
Simplify
[tex]=\dfrac{20}{2}[/tex]
And simplify again.
[tex]=10[/tex]
The halfway point of 4 and 16 is 10. Let me know if you need any clarifications, thanks!
~ Padoru