A total of 60 students are travelling in a bus. The ratio of the number of boys to that of girls is 2 : 1. Then, 15 boys get down and 5 girls get in the bus at the first stop, and 5 boys get in and 10 girls get down from the bus at the second stop. What is the ratio of the number of boys to that of girls in the bus after the second stop?
1
2 : 1
2
3 : 2
3
3 : 1
4
2 : 3