Recycling a flashlight
The equation is (Vsupply - Vf)/If, which works out to 668 ohms (20-3.3)/0.025.  The next-higher 5% standard resistor value is 680 ohms but you're dissipating almost half a watt in the resistor.  They do make LED replacements that have the same base as the original bulb and have the chip at the correct position for the reflector to work, in case you weren't looking for a science project.