Answer:
a) Number of projects in the first year = 90
b) Earnings in the twelfth year = $116500
Total money earned in 12 years = $969000
Step-by-step explanation:
Given that:
Number of projects done in fourth year = 129
Number of projects done in tenth year = 207
There is a fixed increase every year.
a) To find:
Number of projects done in the first year.
This problem is nothing but a case of arithmetic progression.
Let the first term i.e. number of projects done in first year = ![a](https://tex.z-dn.net/?f=a)
Given that:
![a_4=129\\a_{10}=207](https://tex.z-dn.net/?f=a_4%3D129%5C%5Ca_%7B10%7D%3D207)
Formula for
term of an Arithmetic Progression is given as:
![a_n=a+(n-1)d](https://tex.z-dn.net/?f=a_n%3Da%2B%28n-1%29d)
Where
will represent the number of projects increased every year.
and
is the year number.
![a_4=129=a+(4-1)d \\\Rightarrow 129=a+3d .....(1)\\a_{10}=207=a+(10-1)d \\\Rightarrow 207=a+9d .....(2)](https://tex.z-dn.net/?f=a_4%3D129%3Da%2B%284-1%29d%20%5C%5C%5CRightarrow%20129%3Da%2B3d%20.....%281%29%5C%5Ca_%7B10%7D%3D207%3Da%2B%2810-1%29d%20%5C%5C%5CRightarrow%20207%3Da%2B9d%20.....%282%29)
Subtracting (2) from (1):
![78 = 6d\\\Rightarrow d =13](https://tex.z-dn.net/?f=78%20%3D%206d%5C%5C%5CRightarrow%20d%20%3D13)
By equation (1):
![129 =a+3\times 13\\\Rightarrow a =129-39\\\Rightarrow a =90](https://tex.z-dn.net/?f=129%20%3Da%2B3%5Ctimes%2013%5C%5C%5CRightarrow%20a%20%3D129-39%5C%5C%5CRightarrow%20a%20%3D90)
<em>Number of projects in the first year = 90</em>
<em></em>
<em>b) </em>
Number of projects in the twelfth year =
![a_{12} = a+11d\\\Rightarrow a_{12} = 90+11\times 13 =233](https://tex.z-dn.net/?f=a_%7B12%7D%20%3D%20a%2B11d%5C%5C%5CRightarrow%20a_%7B12%7D%20%3D%2090%2B11%5Ctimes%2013%20%3D233)
Each project pays $500
Earnings in the twelfth year = 233
500 = $116500
Sum of an AP is given as:
![S_n=\dfrac{n}{2}(2a+(n-1)d)\\\Rightarrow S_{12}=\dfrac{12}{2}(2\times 90+(12-1)\times 13)\\\Rightarrow S_{12}=6\times 323\\\Rightarrow S_{12}=1938](https://tex.z-dn.net/?f=S_n%3D%5Cdfrac%7Bn%7D%7B2%7D%282a%2B%28n-1%29d%29%5C%5C%5CRightarrow%20S_%7B12%7D%3D%5Cdfrac%7B12%7D%7B2%7D%282%5Ctimes%2090%2B%2812-1%29%5Ctimes%2013%29%5C%5C%5CRightarrow%20S_%7B12%7D%3D6%5Ctimes%20323%5C%5C%5CRightarrow%20S_%7B12%7D%3D1938)
It gives us the total number of projects done in 12 years = 1938
Total money earned in 12 years = 500
1938 = $969000