More arithmetic serious, and progression

mattgad

New member
Joined
Oct 21, 2005
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In the first month after opening, a mobile phone shop sold 280 phones. A model for future trading assumes that sales will increase by x phones per month for the next 35 months.

(280 + x) sold in the second month
(280 + 2x) sold in the thrid month and so on.

Using the model, with x = 5, calcuate:

a) The number of phones sold in the 36th month

a = 280
d = x
n = 36

a + d(n - 1)
280 + 35x

Sub in x = 5, 280 + 35(5) = 280 + 175 = 455

b) The total number of phones sold over the 36 months.

n = 36 a = 280 d = 5
Sn = 18(560 + 175) = 13230

c) The shop sets a sales target of 17000 phones to be sold over 36 months.
Find least possible value of x to achieve this target.

Sn = 17000 a = 280 d = ? n = 36

17000 = 18(560 + 35x)
17000 = 10080 + 630x
630x = 6920
least value of x = 10.9841

Can anyone confirm my answers are correct?

Thanks alot.
 
They all look good to me, except I might round c to 11.
 
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