In one week there will be 50+20=<<50+20=70>>70 seashells
In two weeks there will be 70+20=<<70+20=90>>90 seashells
In three weeks there will be 90+20=<<90+20=110>>110 seashells
In four weeks there will be 110+20=<<110+20=130>>130 seashells
#### 130