A, let Xkg of water be added, and there will be 0.8 * a/100 = a/(100+x).

The solution is X=25.

Therefore, adding 25 kilograms of water to crystal drinks can reduce the sweetness to 80%.

Second, the sweetness of the new beverage is ((a+c)/(100+b))/(a/100) =100 (a+c)/(a (100+b).

That is, the sweetness of the new beverage is100 (a+c)/(a (100+b)) *100% of that of the crystal beverage.