A mixer used in labs can approximated to be a cylindrical vessel of diameter 15 cm and height 21 cm with a rotating cylindrical rod of diameter 5 cm and height 21 cm. To avoid spilling 25% of the net volume needs to be left empty. How many times should the mixer be run to produce 20 liters of the mixture?
1
7
2
8
3
6
4
9
5
10