You calculated the maximum power in each resistor separately so their currents were different. But in the circuit the resistors are in series so the current in each one must be the same.
Calculate the maximum power of each resistor then calculate its current. Then with each resistor using the current that is the smallest, calculate the total voltage.
It is a silly exercise because NOBODY uses a resistor at its maximum allowed power rating when it is almost red hot, it might desolder itself, it will melt any plastic capacitor or case nearby and will be even hotter when enclosed/and or in summer.