Answer:
56 oz
Step-by-step explanation:
He stops 7 times in 21 miles because he stops every 3 miles and 21 divided by 3 equals 7. He also drinks 8 oz of water every time he stops, and if he stops 7 times then we just multiply 8 and 7, and since 8×7=56 he drank 56 oz of water after 21 miles.