Answer: 4 books maximum
Reason:
x = number of textbooks
650x = weight of all x books, at 650 grams each
3 kg = 3000 g
650x = 3000
x = 3000/650
x = 4.615 approximately
Round down to the nearest integer. This is because 4 textbooks weigh 4*650 = 2600 grams = 2.6 kg; while 5 textbooks weigh 5*650 = 3250 grams = 3.25 kg which is over the weight limit.
This means 4 books is the most the rucksack can hold. This is of course only when we have 4 of the same exact book.
