All three windows are the same size.
A has 10 complete waves visible through the window. B has 3, and C has 4.
So A must have the smallest wavelengths.
One simple way to losing weight is by calorie counting. Peter can go to his nutritionist and learn just how much calories he can have as intake per day to lose weight. The nutrition label on any food item lists the calories and their serving sizes. This can be used to keep track of how much calories Peter has taken each day, so as to not go over his daily limit.