Question

How to calculate the highest 9 digit number using 8,7,6,5? How to calculate similar problems using different combinations?

Answer 1

Since no limit of using a particular number is given, we have to consider lots of cases.
Case 1:
any number can be used any number of times
then the largest number will be the largest number repeated 9 times.
In this case it is 888888888
Case 2:
if a number should be used atleast once, then the largest possible number is
888888765
Case 3:
if a number should not be used for more than a maximum number of times, then
the largest should be placed initially followed by the other numbers
for eg if a number shouldnot be used for more than 3 times, then in this case we can write
8887776665

similarly lots of combinations and cases could be considered, unless a particular condition is specified in the question,.