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 mat
h 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?
