C ( To make things simple, let's turn 80% into a fraction, 8/10. To get a common denominator let's turn 8/10 into 4/5. Since they are both of equal size, the first group would have 2/5 left-handed people and the second 1/5 left-handed people. Let's imagine that each group has 5 people in it. There would be 10 people in total. There is 1 person for each fifth so we can add now. 4 + 3 = 7 right handed people. To check, 2 + 1 = 3 and 3 + 7 = 10. so 7/10 of the class is right-handed. )
