In
a bag there are 80 balls, of which some are blue and some are red. The
ratio of the number of blue balls to that of red balls is 1 : 3. If some
red balls are replaced with blue balls, the ratio of the number of blue
balls to that of red balls becomes 3 : 5. How many red balls are
replaced by blue balls?

1) 20
2) 10
3) 30
4) 40