How do you figure out the amount of cement for a sidewalk by 80 pound bag?

A U.S. bag (or sack) of cement contains 94 lbs. Note this is for cement not premixed concrete where you just add water. The origin of 94 lbs/sack for cement comes from 1 cubic foot of loosly bagged...

1 94-pound bag of cement is approximately equal to 1 cubic foot.

http://www.google.com/products?hl=en&q=concrete%20mixIn Pakistan 80 lb bag is not available. however it will cost about US$ 3.5 for 80 lb of cement.

The weight of a 94 lb. sack of cement is incidental - the bag is packaged in that size in order to hold one cubic foot of cement, and 94 pounds is simply what one cubic foot of dry cement powder...

Answer: 6 pints = 3 quarts of water. Note: Don't add all the water at once. Put in 2 quarts or so, mix and adjust while mixing. It's easy to add more water --- impossible to add LESS water. Good...