The first way I do this, is transform the 12V and 16V voltage sources into current sources with parallel resistor, which gives me (after combining parallel resistors and current sources) R_equivalent = 5.714 ohm and current source of 4 amps, which is equivalent to voltage source of 22.85 volts with the same R_equivalent
So, answer is: 22.85 volts with 5.714 ohm resistance in series.
However, I have tried something else more complicated just to see if it would work, but It's giving me another result. I begin by transforming the 3 amps current source into a 30V voltage source in series with the 10 ohm resistor. Then I transform the 12V and 16V voltage sources into current sources with parallel resistance. All these 4 elements are thefore in parallel. 0.6 Amps current source, 0.4 amps current source, 20 ohms resistor, 40 ohm resistor.
Combining the two resistors I get one single resistor of 13.33 ohms, and the current sources are added to give 1 Amp. The current source and the resistor are in parallel. I transform this to voltage source of 13.33 Volts with resistance of 13.33 ohm
We now have 30V voltage source, 10 ohms resistor, 13.33 volts voltage source, 13.33 ohms resistor, all in series. Simplifying, this gives 43.33 volts voltage source, 23.33 ohms resistor. Which is not the same as the first answer.
What did I do wrong?
Thank you very much.