hi,
You have to use a formula which is known as of binomial coefficient which is :
k among n is : C ( n /k) = n! / k! ( n-k)!
! = factorial
ex : 5 object among 8 is : C ( 8/5 ) = 8 ! / 5! * ( 8-5) !
As 8! = 8*7*6*5*4*3*2*1 = 40 320
As 5! = 5*4*3*2*1 = 120
as 3 ! = 3*2*1 = 6
then : 40 320 / 120 * 6
40 320 / 720 = 56
Conclusion : there is 56 way to select 5 object among 8 of them.
ps : you can also use Pascal Triangle to do such a thing.
Regular pentagon means the shape with 5 sides which their lengths are equal to each other.
so, if the total perimeter of the pentagon is 10s - 20,
we can find the length of each side by dividing it by 5.
(10s - 20) / 5
we can expand this equation by dividing each number in the bracket by 5
= 2s - 4
Therefore, the answer should be D.
I'm not 100% sure, but I'm pretty sure its 11 and 13
