Question 1197648
In preparing a bid for constructing a new building,a contractor determines that the foundation and basement will cost $605000 and the first floor will cost $360000.Each floor above the first will cost $15000 more than the one below it.How much will the building cost if it is to be 18 floors high? 
Thank you.
<pre>To determine the cost of the 1<sup>st</sup> floor up to the 18<sup>th</sup>, we use the formula 
for the SUM of an AP: {{{matrix(1,3, S[n], "=", (n/2)(2a[1] + (n - 1)d))}}}
                     {{{matrix(1,3, S[18], "=", (18/2)(2("360,000") + (18 - 1)*"15,000"))}}}, with n being 18, a<sub>1</sub>, 360,000, and d, 15,000
                       S<sub>18</sub> = 9[720,000 + 17(15,000)]
                           = 9(720,000 + 255,000)
                           = 9(975,000) 
                           = 8,775,000
With cost of the 1<sup>st</sup> floor up to the 18<sup>th</sup> being $8,775,000, and cost of foundation and basement, $605,000, 
ENTIRE building then costs: 8,775,000 + $605,000 = <font face = "consolas" color = "red" size = 3><b>$9,380,000
BTW, solution by tutor @EWATRRR is INCORRECT!</font></b>