Proportion: A statement that lines two ratios 

Variable: A Symbol for a value we don't know yet. It is usually a letter (e.g. x, y). 

Approximation: A result that is not exact, but close enough to be used.

Scale: The comparison of something drawn to its actual size.

Scale Factor: A scale factor is when you enlarge a shape and each side is multiplied by the same number. This number is called the scale factor. 

Currency: Money, such as coins or banknotes, used as a medium of exchange.
(Click here to see the top 10 most beautiful world currencies) 

Conversion graphs: are straight line graphs that show a relationship between two units and can be used to convert from one to another. 

Direct proportional graphs: are straight line graphs to show a proportionality relationship. As one value increases, so does the other value. It always passes the origin (0,0). Conversion graph is a type of direct proportional graph. 

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Command words:   

Convert: to change a value or expression from one form to another