Combined series and parallel resistors -- adding resistors
If R2 and R3 were in parallel, their combined resistance would be 50 ohms. This in series with the R1 100 ohm resistor would make a combined resistance of 150 ohms. But this is not what is happening.

R2 and R3 are each in series with an LED. These resistors, in combination with R1 are limiting the current through the LEDs. So the usual series/parallel resistor analysis is not applicable due to the presence of the LEDs.