The difference in width between where they're starting from and their destination is 80-50 = <<80-50=30>>30 yards
The width increases 2 yards every 10 m along the river so they would have to cover (30/2)*10 = <<(30/2)*10=150>>150 meters
Traveling at the rate of 5m/s, he will spend 150/5 = <<150/5=30>>30 seconds
#### 30