Answer:
Let the number of people that wore black pants be

Let the number of people who wore blue pants be

The total number of students in the choirs is

Three times as many students wore black pants as blue jeans can be represented below as

The total number of students will be represented below as

Substitute the equation (1) in equation (2)

Hence,
The number of students who wore blue pants is =11
The right answer is OPTION D