Answer: 
 Large box: 

 Small box: 

Explanation:
 Let be "l" the weight in kilograms of a large box and "s" the weight in kilograms of a smalll box.
 Set up a system of equations:
 

 In order to solve the system of equations, you can apply the Elimination Method. Multiply the first equation by -2, add the equations and then solve for "l":
 

 Finally, susbstitute the value of "l" into any original equation and solve for "s":
 
