For twelve people you need: 2 litres of lemon-lime soda, 1 pint of sherbet and 3 cups of ice tea. For 36 people you need: 6 litres of lemon-lime soda, 3 pints of sherbet and 9 cups of ice.
Because logarithms were a lot less time-consuming than other methods of finding roots. They were also more accurate in the sense that it was harder to make a mistake.
