7 liters =(7000) ml, 9000 ml =(9) liters.

4 l =(4000) ml， 10000 ml =( 10) l，

20 l =(20000) ml, 80000 ml =(80) l,

12L =( 12000) mL，( 17000) mL = 17L。

Mathematics (English: Mathematics, from the ancient Greek μ θ η μ α (má th ē ma); Often abbreviated as math or maths]) is a discipline that studies concepts such as quantity, structure, change, space and information, and belongs to a formal science from a certain point of view.

Mathematics is produced by counting, calculating, measuring and observing the shape and motion of objects through abstract and logical reasoning. In many countries and regions, mathematics has become a part of education. It is applied in different fields, including science, engineering, medicine, economy and finance. Mathematicians also study pure mathematics, which is the substantive content of mathematics itself and does not aim at any practical application.

Mathematics originated from the early production activities of human beings, and the ancient Babylonians had accumulated some mathematical knowledge, which could be applied to practical problems. As far as mathematics itself is concerned, their mathematical knowledge is only obtained through observation and experience, and there is no comprehensive conclusion and proof, but their contribution to mathematics should also be fully affirmed.