Answer
The number of pages will be 1500 by the $19th$ edition.
Work Step by Step
Using the model found in part (a), $L=55n+475$
we solve for n when $L=1500.$
$1500=55n+475\qquad/-475$
$1025=55n\qquad/\div 55$
$18.6\approx n$
The number of pages will be 1500 by the $19th$ edition.