calculated Voltage explanation
If two resistors are in series then they have the same current. Also, the \$30mA\$ current source is in series with the \$2k\Omega\$ and \$4k\Omega\$ resistors. Therefore the \$4k\Omega\$ resistor has \$30mA\$. By Ohm's Law \$4k\Omega * 30mA = 120V\$. Likewise for the \$2k\Omega\$ resistor: \$2k\Omega * 30mA = 60V\$.