\in the spring of 2009, the first annual fishing competition was held in southern california in which the anglers who caught the largest halibut (by weight) shared prize money. two of the heaviest halibut caught in the contest weighed 31 lb. and 17 lb. 2 oz. what was their difference in weight

The difference in the weight of the sea creatures captured by the fishermen can be calculated by subtracting the smaller weight or lighter weight to the heavier one. Since, the lighter weight involves the unit of oz, 1 unit of the heavier weight should also be converted to this unit.
The conversion factor that should be used here is 1 lb = 16 oz.
                           difference = (30 lb + 16 oz) - (17 lb + 2 oz)
                           difference = 13 lb + 14 oz
Thus, the difference in the weights is equal to 13lb 14 oz. 

C. expression because any expression or equation may contain more than one variable