A 20 g ball is fired horizontally with initial speed vi toward a 100 g ball that is hanging motionless from a 1.0 m long string.

A 20 g ball is fired horizontally with initial speed vi toward a 100 g ball that is hanging motionless from a 1.0 m long string. The balls undergo perfectly, elastic collision, after which the 100 g ball swings out to a maximum angle of 50 degrees. What was vi?