A business has 4 humidifiers. The cost (in dollars) to buy and maintain the humidifiers for n weeks is represented by 640+40n. Write an expression
that represents the cost to buy and maintain one humidifier for n weeks.

White shapes and black shapes are used in a game. some of the shapes are circles. while the rest are squares. the ratio of the number of white shapes to the number of black shapes is 5: 11. the ratio of the number of white cicrles to the number of white squares is 3: 7. the ratio of the number of black circles to the number of black squres is 3: 8. work out what fraction of all the shapes are circles.