Answer :
[tex]x-\ Lisa\\
y-\ Chris\\\\
x+y=25.50\\
x-13.5=y\\\\
x+x-13.5=25.50\ \ \ | add\ 13.50\\\\
2x=39\ \ \ | divide\ by\ 2\\\\
x=19.50\\\\ Lisa\ spent\ 19.50 \$[/tex]
I think you just subtract the total - the money lisa spent
lets try it
25.50$-13.50$ = 12$
so we all know that its 12$
wanna PROVE IT , try adding 12$+13.50$ =
it will give us the total ;p