7/9 is the answer I believe
I don't have my calculator handy just now. But just looking at that problem, why don't you try +20 and -45 and see if they do the job.
Answer:
This can be found by using the permutations and combinations formuals.
Combinations being the C equation and Permutations being the P equation.

Explanation:
A permutation is used for the list of data (where the order of the data matters) and the combination is used for a group of data (where the order of data doesn't matter).