This would be a combination I believe not a permutation so the equation to use would be nCr= n!/(n-r)!r!
So 10C2=45
I believe the equation shown above is an example of:
B. Associative Property of Addition.
Hope I helped!!
(20:4)x3=15 (Ellas grade)
(20:5)x4=16 (Minhs grade)