Let's call the number of pages that Jen wrote "j".
then tom (t) wrote:
What we know:
j=10t
this means:
t=0.1 j
and j+t=396
for this it's practical to substitute j with 10 t:
10t+t=396
11t=396
we divide both sides by 11:
t=36
so tom wrote 36 pages, and Jen wrote ten times as much, or 360 pages (that's a lot!)