From the condition, you have these two equations


    T + M = 20       (1)    (T is the number of the T/F-questions);

   3T + 11M = 100    (2)    (M is the number of the multiple choice questions)


From equation (1), express T = 20-M and substitute it into equation (2)


    3*(20-M) + 11M = 100.


Simplify and solve for M


    60 - 3M + 11M = 100

    8M = 100 - 60 = 40

     M            = 40/8 = 5.


ANSWER.  5 multiple choice questions and  20-5 = 15  True/False questions.