2012 AMC 12B Question 7

Small lights are hung on a string $6$ inches apart in the order red, red, green, green, green, red, red, green, green, green, and so on continuing this pattern of $2$ red lights followed by $3$ green lights. How many feet separate the $3$rd red light and the $21$st red light?

Note: $1$ foot is equal to $12$ inches.