In order to convert the pressure from mmHg to mmH20 (that is, using water instead of mercury), we need to use the following relation:
1 mmHg = 13.6 mmH20
(This relation can be found by calculating the ratio of the densities)
So we have:
[tex]245\text{ mmHg}=245\cdot13.6\text{ mmH20}=3332\text{ mmH20}[/tex]Therefore the water must be 3.332 meters tall.