What is the Vout of 74154 demux and 7404 hex inverter?
You may be looking at the Absolute Maximum ratings, rather than Operating Characteristics. 5.5 volts is the maximum voltage that can be be applied to an output (or any) pin without damaging the IC. 

For the 7404, the Operating Characteristics table shows the minimum Voh (output high voltage) is 2.4 volts, and typical is 3.5 volts when sourcing 0.4 mA.

Vol (Output Low voltage) is 0.2 V typical to 0.4V maximum when sinking 16 mA.