Answer: 21 feet every 3 seconds
Explanation:
21 feet every 3 seconds is faster than 63 feet every 12 seconds because if you were to make 3 seconds catch up to 12 seconds, you'd multiply 4 to 3 which will get you 12. Then, you would multiply 4 to 21 which would give you 84. 84 feet every 12 seconds is faster than 63 feet every 12 seconds because 84>63.
21 feet every 3 seconds
3*4=12
21*4=84
84 feet every 12 seconds > 63 feet every 12 seconds.