oldest: 48 pages / 60 mins = 0.8 pages / min
middle: 18 pages / 14.4 mins = 1.25 pages / min
youngest = 5 pages / 4 mins = 1.25 pages / min
The middle brother and the youngest brother are reading at the same constant of proportionality.
let x = minutes spent reading.
let y = total pages read.
y = 1.25x