Oliver works at a book store. he packed 20 identical paperbacks and 9 identical textbooks in a box .the total mass of the book was 44.4 pounds . after he put 1 more textbook and 5 more paperbacks in the box, the total mass of the books was 51 pounds.

Let x be the mass of the paperbacks and y be the mass of the textbook.

20x + 9y = 44.4 (1)
25x + 10y = 51 (2)

(1) x 10:
200x + 90y = 444 (1a)

(2) x 9:
225x + 90y = 459 (2a)

(2a) - (1a):
25x = 15
x = 0.6 sub into (1)

20 (0.6) + 9y = 44.4
12 + 9y = 44.4
9y = 44.4 - 12
9y = 32.4
y = 3.6 

So the paperback's mass is 0.6 pounds and textbook is 3.6 pounds