The length $m$, in inches, of a model train is directly proportional to the length $r$, in inches, of the corresponding real train.

(a) Write an equation that expresses $m$ as a function of $r$, using $k$ for the constant of proportionality.
help (equations)

(b) An HO scale model train is 1/87th the size of a real train. What is the constant of proportionality?
help (numbers)
What is the length, in feet, of a real locomotive if its HO scale model is $10$ inches long?
feet. help (numbers)

(c) A Z scale model train is 1/220th the size of a real train. What is the constant of proportionality?
help (numbers)
What is the length, in inches, of a Z scale model if its real locomotive is is $60$ feet long?
inches. help (numbers)