Question

A bench is made of a 3 meter long plank with a mass of 12kg and 2 sawhorses(each 0.2m from an end).There are two boxes on the bench. A 15kg box is 0.8 meter from the left hand side. A second box is 0.5 m from the right hand side. What would the mass of the second box have to be for the normal force exerted by the right sawhorse to be 3 times larger than that exerter by the left sawhorse?