The answer is 19 because u have to put the number from least to greatest and then cancel one number from each side everytime and then there was two numbers left in the middle (18 & 20) between those two numbers is 19.
1 kilometers =1000 meters 0.86km=0.86*1000=860m John swam 860 meters each day. 1 week = 7 days 3weeks=3*7=21 21*860=18060 m Therefore he swam 18060 meters in those three weeks.
