HELP confused on resistors and leds?

If batteries give 1.5V each, four will together give 6V. You can connect 3 LED's in series with a resistor of 60 (or 56) ohms. You require seven such sets to light up 21, as 20 is not divisible by 3. Otherwise, have 2 LEDs in one set and increase the resistor to 140 (or 120) ohms.