30cm is the difference between the height of the soap in the full dispenser and height of the remaining soap.
The formula for cylindrical height is:
Area
-------- - r
2 * π * r
Cylindrical height of full dispenser.
5652 5652
--------------------- - 7.5 = ------------ - 7.5 = 120 - 7.5 = 112.50
2 * 3.14 * 7.5 47.1
Cylindrical height of dispenser with lesser liquid soap
4239 4239
-------------------- - 7.5 = ------------ - 7.5 = 90 - 7.5 = 82.50
2 * 3.14 * 7.5 47.1
Cylindrical height of full soap 112.50
Cylindrical height of lesser soap - 82.50
Difference in height of soap content 30.00