Question 717985
Let x = number of trips for the 13 ton truck
Let y = number of trips for the 15 ton truck.


x+y=18, counting the trips.
13x+15y=248, compute for tons of gravel hauled.


Those two can certainly well be graphed.  A calculated solution should be prefered for the intersecting (x,y) point.  The graph suggests  11 trips.


{{{graph(450,450,-1,18,-1,18,-x+18,-(13/15)*x+248/15)}}}
y=-x+18, y=-(13/15)*x+248/15)