Simple electrical circuit calculation problem
The voltage is:

\$U_0 = U \cdot {{R \cdot n} \over {5 \cdot R}} = U \cdot {n \over 5}\$, where \$n = \$ the point where voltmeter is connected.  

So in your case:

\$U_0 = 30 \cdot {{n} \over {5}} = 6 \cdot n \space Volts \$

If you split R between 1 and 2 in half you get:

\$U_0 = 30 \cdot {{1.5} \over {5}} =  {45 \over 5} = 9 V\$