As we all know, there are seven days in one week.
To do this we should make a proportion, with numbers of sodas on top and days on bottom. X will be the # of sodas in a week.
6/2 = x/7 < our proportion
Then, we cross multiply
42 = 2x
21 = x
x = 21, or in other words Sally drinks 21 sodas in a week.