The average weight of 20 students of a class is 60 kg. A student whose weight was 45 kg left the class and new student replaced the old one. Hence, the average weight of students increased by 1 kg. Find the ratio of old and new student's weight.
1
9 : 13
2
13 : 9
3
8 : 7
4
7 : 8
5
None of these