Answer:
210 x 21 = 4410 calories burned for 1 adult
4410 x 8 = 35280 calories burned for 8 adults
Explanation:
First you figure out how much calories are burned total for 1 adult on a 21 hour bike ride with the information that you know of how much adults burn.
Next you multiply your answer by 8 to figure out the total of calories burned for all 8.