If,
5 + 3 = 835
2 + 4 = 642
7 + 0 = 707
Then,
6 + 1 = ???
6 + 1 = (6+1){Sum of the two digits}(1){2nd digit}(6){1st digit} = 716.
5+3=8 then right to left 35 so 835
ans:716