820 is the number exactly between 819 and 821.
819+922=1731
819 miles between the borders
it seems like your equation is this... 400 - (821 + 77) = ? your answer would be -498 negative 498
821 x 1 = 821
21% of 819 = 819*21/100 = 171.99
819
819 is a natural number
819-13 = 806
819 ÷ 39 = 21
819 is DCCCXIX
50% of 821 is 410.5
821 miles