To get maximum sum we need to find the largest possible 3 digit number from these number
givennumber are
1,5,6,7,8,9
One of the largest 3 digit number is 987
Now from the other three number
That is 1,5,6 , we want to make largest number
It willbe 651
So
We got two 3 digit no
Now sum
987+
651
_____
1638