What does cross-sectional area mean in math term?

The only math term I can think of is geometry and it means when a solid is cut through parallel to the base.
EDIT: After further research since it was all I could think of. This is the only math term for what you are looking for.

When i cut right through the middle of an object

The ratio of 3:5:2 mean 10 gallons of paint from what quantitie 2 gallons are wite
so total needs 35 gallons so this mean 3*10 +5 gallons 
from these 10 gallons 2 gallons are white so from 3*10 gallons will be 3*2 gallons white what is 6 gallons white 
but there are again 5 gallons till 35 gallons so than we know that from 10 gallons has 2 gallons white so from this result that from 5 gallons will be 1 gallon white 
so these above resulted 6 gallons white plus assumed with this 1 gallon white so total will be 6+1=7 gallons of white paints she need