And for (very simple) calculations - according to Kirchhoff's voltage law the voltage on the resistor is 5.6-2.2 (all voltages in the circuit together will be equal to the source voltage), which means 3.4V (indicated in yellow on the right). If the voltage on the resistor is 3.4V, and the current passing through it is 20mA (indicated in yellow in drawing) according to the current law then the resistance of the resistor (according to Ohm's law) is 3.4 / 0.02 = 170Ω. Since there is no standard resistor of 170Ω, we have the option of using either 160N or 180N. A resistor of 160 will give us a final voltage too low (and will drop more voltage on the LED than we want, which is likely to cause damage) and so a resistor of 180Ω is chosen.
Tuesday 10 September 2019 6:55am