We can see here 2 cases,
- If she attempt 1st Qa , then Qb
- If she attempt 1st Qb and then Qa
Qa = Attempt right , Qa’ = incorrect attempt ;
Qb = Attempt right, Qb’ = incorrect attempt
Now we see this two cases by tree method ----
Case 1:
Marks for both correct = (10 + 20) = 30
Only for Qa correct = 10 marks
So, Expected marks = (0.8 x 0.5 x 30) + (0.8 x 0.5 x 10) = 16
Case 2:
Marks for both correct = (10 + 20) = 30
Only for Qb correct = 20 marks
So, Expected marks = (0.8 x 0.5 x 30) + (0.5 x 0.2 x 20) = 14
So, we can make this conclusion that, Case 1 expected marks > Case 2 expected marks
So, She should do first Qa and then Qb Expected marks 16.
Hence, option D is correct