Group a contains 157 students. of these, 51 students in group a have taken math 2a, 80 have taken math 2b, and 70 have taken math 2c. 15 have taken both math 2a and 2b, 20 have taken both math 2a and 2c, and 13 have taken both math 2b and 2c. how many students in group a have taken all three classes?