0is less than or equal to 2x - 10 is less than or equal to 20

What is the maximum total area that 450cm of string can enclose if it is used to form perimeters of two adjoining rectangles as shown?

Name the most appropriate metric unit for each measurement like the mass of cow

How many kilograms of a 5% salt solution and how many kilograms of a 15% salt solution must be mixed together to make 45kg of an 8% salt solution?