3 sevenths 2 sevenths equals

3 sevenths plus 2 sevenths equals 5 sevenths

3/7 + 2/7 = 5/7 because you're adding 3 and 2 which gives you 5.

The answer is 5 sevenths because 3 + 2 = 5.

To properly make the comparison between the weights of the books, we compute for their weights in similar units. For this item, we need to convert the weight of the first book to kilogram.
             conversion:
                             Book 1 :    (7.49 lbs) x (0.4535924 kg / 1 lb) = 3.397 kg
Since the second book weights 3.91 kg, it is heavier than the encyclopedia. Subtracting the weight of the first book from the second one will give us an answer of 0.51 kg. Thus, the answer to this item is letter D.