What is the equivalent resistance of a circuit that contains four 75.0 resistors connected in series to a 100.0 v battery

The equivalent resistance of several resistors connected in series is
the sum of their individual values.

Four 75-ohm resistors connected in series are electrically equivalent
to a single resistor of    300 ohms.

The battery voltage doesn't matter.  In fact, it doesn't matter whether the
resistors are anywhere near a battery or not.  They could just as well still
be in the little Radio Shack envelope on the top shelf of the storage room. 
If they're connected in series, then they're still electrically equivalent to a
single 300-ohm resistor.