hi mike, i am using series parallel arrangement, i just though of doing 2 seperate strings before you put your reply. should have thought of this earlier on really
No, that is where you are getting confused. If I have 5 LEDs in series and those LEDs are 20 mA the current for the string is 20 mA and not 100 mA. Forget resistors for a moment. Let's say I have 5 each LEDs rated as 1.2 volts @ 20 mA. If I place 5 of them in series I will need a voltage source of 1.2 * 5 = 6 and a current of 20 mA. Does that make sense to you now?
Things are seldom perfect but nice round numbers work well for examples.
Actually in the real world you would not want to run a LED at its maximum current. Running a little below is the way to go for a nice long life. Additionally when choosing a resistor to limit the current to a LED or LED string you likely won't find a resistor of the exact value the math delivers. Short of special purpose or precision resistors you would use a generic value one size up from what the numbers call for. For example a number like 40 Ohms isn't common but 47 Ohms is so you would go with a 47 Ohm resistor rated for the proper wattage.
No LEDs have a characteristic voltage drop of 1.2v. The minimum is 1.7v for red LEDs. About 1.9 to 2.2v for green and other colours. White is about 3.4v to 3.6v.