The area of a parallelogram is given by

Now, if we consider the 9.9 inches side as the base, then the height is the one labeled with 5.5 inches.
If instead we choose the 11 inches side as the base, the height is h.
So, we can express the area in this two equivalent ways:

Solving for h, we have

1st rectangle:
width: 2.48cm
length: 4.96
2nd rectangle:
width: 4.96 (equals to the length of the 1st rectangle)
area: 9.92
length: 9.92/4.96 = 2
D because they all increase by the same number (+1)
3 gallons of acid = 100% concentration acid
? (water+acid) gallons = 15 concentration acid
Note: Every gallon of water added reduces the percentage concentration of acid. Therefore, the relationship is inverse proportionality.
Solution:
Total number of gallons (water+acid) = (3*100)/15 = 20 gallons
Therefore, gallons of water = 20-3 = 17 gallons