(a) 15
The seven boys who failed the test scored the marks less than these given marks. Let the marks be x1 , x2 , x3 , x4 , x5 , x6 , x7
Then the marks arranged in ascending order are x1 , x2 , x3 , x4 , x5 , x6 , x7 , 12, 15, 15, 15, 16, 17, 17, 18,18, 19, 19, 19
∴ Median score = \((\frac{19+1}{2})\)th item
= 10th item = 15.