linear programming- urgent

A company produces two kinds of concrete: concrete A and concrete B. Both concrete A and B are produced by mixture C and mixture D but in different composition. A barrel of mixture C contains 20% rock and 25% water and the cost of a barrel is $35. A barrel of mixture D contains 25% rock and 15% water and the cost of a barrel is $40. The weekly demand for the concrete A is at least 26 barrels and demand for the concrete B is at least 34 per barrels. At least 23% of each barrel of concrete A must contain rock and at most 25% of each barrel of concrete B should contain water. Now, in order to decide how many barrels of mixture C and D to buy for each week for mixing, and to satisfy demand at minimum cost, formulate a linear programming. (No need to solve the model)

Sorry! I do NOT know the meaning of the following sentence:
At least 23% of each barrel of concrete A must contain rock and at most 25% of each barrel of concrete B should contain water.
Please clarify.