Why is the resistance of parallel circuits lower than either of the individual parts
The current through the 150 Ohm resistor will be 0.2 Amp, whether the 100 Ohm resistor is there or not, and the current through the 100 Ohm resistor will be 0.3 Amp, with or without the 150 Ohm resistor.
With both resistors present, the total current will be 0.5 Amp.  To get that current with a single resistor, you would need a 60 Ohm resistor, by Ohm's Law.