Put 60KG and 40KG in boat. GO. 
Put them on shore and put 60KG back in boat. GO. 
Put 60KG back on shore. Put 80KG in boat. GO. 
Put 80KG back on shore. Put 40KG in boat. GO. 
Put 40KG and 60KG in boat. GO. 
Put both back on shore. Put 40KG back in boat. GO. 
Put 40KG back on shore. Put 90KG in boat. GO. 
Put 90KG on shore. Put 60KG in boat. GO. 
Put 40KG in boat with 60KG. GO. 
The end!