Question

Skyscraper Windows

A building is 12 stories high and is covered entirely by windows on all four sides. Each
floor has 38 windows on it. Once a year, all the windows are washed. The cost for
washing each window depends upon the window's floor:
Floor
Cost per Window
$2.00
$2.50
$3.00
For example, each window on Floor 2 costs $2.50 to wash. This pricing scale continues
for the windows on each floor.
1. How much will it cost to wash all the windows of this building?
2. If the building is 30 stories tall, how much will it cost to wash all the windows?
3. If the building is n stories tall, how much will it cost to wash all the windows?

Answer 1

A.) $2,166

B.) $10,545

C.) (n/2)(3.5 + 0.5n) × 38

Explanation:

Floor - - - - - - - - - - 1 - - - - - - - 2 - - - - - - - 3

Cost / window - - $2.00 - - - $2.50 - - - $3.00

Cost of washing first floor = $2 × 38

Cost of second floor is: (2 + 0.5) × 38

Cost of third floor is: (2 + 2 × 0.5) × 38

Cost of n floor : (2 + (n - 1) × 0.5) × 38

Therefore,

Total cost :

(2 + (2 + 0.5) + (2 + 2*0.5) + ... + (2 + (n - 1)*0.5)) × 38

Using Arithmetic progression for the sequence :

2, (2 + 0.5), (2 + 2*0.5),..., (2 + (n - 1)*0.5)

first term 'a' = 2; common difference 'd' = 0.5

Sum of the first n terms of arithmetic sequence terms is:

S = (n/2)(2a + (n - 1)d) ; Where a = 2 and d = 0.5

S = (n/2)(2*2 + (n - 1)*0.5)

S = (n/2)(4 + 0.5n - 0.5)

S = (n/2)(3.5 + 0.5n)

To wash all windows with n stories :

S × 38 windows

A.) To wash all windows : stories = 12

n = 12

(12/2)(3.5 + 0.5(12)) × 38

6(3.5 + 6) × 38 = $2,166

B.) if building is 30 stories tall:

n = 30

(30/2)(3.5 + 0.5(30)) × 38

15(3.5 + 15) × 38 = $10,545

C.) If building is n stories tall :

(n/2)(3.5 + 0.5n) × 38