One for a cold morning: I'd run out of de-icer, and the frost was really tough to scrape off.  When melting the ice off my car I thought if I just add a couple of tablespoons of salt to a quart of warm water - that should both melt the ice and prevent refreezing - as I think sea water freezes at -21°C, and its hardley ever that cold.

No - that wasn't enough salt - It soon refroze. 

What would the correct proportions of salt to water be to get that seawater -21°C freezing temperature?