muck_banner.rb

#banner_print.rb
$tax_rate = 0.15
$colour_rate_breakpoint = 2
$colour_cost_low = 10
$colour_cost_high = 15
$cost_pr_sqft = 15

#single function
def calculate_cost(dim_x, dim_y, num_colours)
  price = calculate_tax(calculate_area_cost(dim_x, dim_y) + colour_cost(num_colours))
end

#Supporting Features
def calculate_tax(cost)
  cost = cost * ($tax_rate + 1)
end

def calculate_area_cost(length, width)
  cost = length * width * $cost_pr_sqft
end

def colour_cost(n_colours)
  cost = n_colours <= $colour_rate_breakpoint ? n_colours * $colour_cost_low : n_colours * $colour_cost_high
end

puts calculate_cost(1,2,3)