Serena added 10 grams of sugar to 20 grams of water to make a sugar syrup.
The total amount of sugar syrup made is the sum of the weight of sugar and the weight of water, so:
10 grams (sugar) + 20 grams (water) = 30 grams (sugar syrup)
To find the percentage of sugar in the syrup, we can divide the weight of the sugar by the total weight of the syrup and multiply by 100.
% sugar = (weight of sugar / total weight of syrup) * 100
% sugar = (10 / 30) * 100 = 33.333 %
So the sugar syrup made by Serena contains 33.333% sugar by weight.
It's important to note that the sugar syrup doesn't have only sugar, it also have water, the sugar is just the main ingredient and the percentage of sugar is just the proportion of sugar within the syrup.