A= {1,2,{3,4},5}
Sir, are {1,2,5} subsets of set A?
Are they elements (belongs to)?

Dear student,
               The formula to find the number of sub sets is (2^n) - 1

n being the number of elements, {-1 to eliminate the null set}

2 power 5 is 2 X 2 X 2 X 2 X 2 = 32

32–1 = 31.
                        Yes, (1,2,5) are  subsets of set A (1,2,(3,4),5)
Regards