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 third red light and the 21st red light? If the answer is x feet, what is the value of 10x?

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

Let's think step by step and output the final answer within \boxed{}.
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