How to adapt a string of 10 LEDs powered by 3 AA batteries to work off a USB port?
You did read it incorrectly, Red|Black|Black is 20 Ohm, not 200.
White LEDs (which are actually blue LEDs with white luminophore) drop about 3V (a multimeter would give you an exact value), leaving about 1.5V to the resistor. This amounts to 75mA of current. To have the same current with a 5V supply, you need a resistor which lets 75mA through at 2V voltage drop, which is around 27 Ohm.