water changes

[Brownskinner]

I just bought a bucket of Kent salt for my tank. I plan on doing a water change but there is no directions on the bucket as to how much salt to mix with the water. I have a 55g tank and plan to change about 20g. From your experience how many (cups) should i use for the 20g's of water??? as well how long should i leave it mixing with a power head for in the rubbermaid container???

Thanks in advance for any input