Well, darling, the difference between 400 and the sum of 821 and 77 is -498. You simply subtract the sum of 821 and 77 (which is 898) from 400. Easy peasy lemon squeezy!
819+922=1731
819 miles between the borders
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
922 - 819 = 103