Allen drank 2/3 liter of water monday before going jogging. he drank 5/7 liter of water after his jog. how much water did allen drink altogether? write your answer as a mixed number this is hard for me

First, change the denominators so that they are the same. 2/3 would be 14/21 and 5/7 would be 15/21. The denominator is 21 since 3 and 7's LCF (least common factor) is 21. Then you would multiply 3 by 7 and 7 by 3. You would do the same on the top, 2 with 7 and 5 with 3. Then you would get 14/21 + 15/21. Just add the numerators and keep the denominators the same. 

Your result will be 1 and 8/21.