i Just checked the answers they dont use the ppm at the start :(
Ahh so simple cant believe i got this wrong.
they found moles(CaC03) which were 10000
.: moles of C02 = 10000
.: Volume = 10000x 24.79
.: Volume to pass in tree = 247900/3.8x10^-4
=6.52x10^8L