Let the pages in the book be x.

Morning = 25
Afternoon = 1/4 (x - 25) = 1/4 x - 25/4
Night = 1/3 (1/4 (x - 25)) = 1/12 x - 25/12
25 + 1/4 x - 25/4 + 1/12 x - 25/12 = x
x - 1/4 x - 1/12 x = 25 - 25/4 - 25/12
2/3 x = 50/3
x = 25

Let the amount of money that Alvin have be x and the amount of money needed to buy the pen be y.

y - x = 2.80
2.80 = 2.20 + 0.60
y - x - 0.60 = 2.20
x + (x + 0.60) = y - 1.70
x + 0.60 + 1.70 = y - x
x + 2.30 = 2.80
x = 0.5