Obviously I'm making some sort of systematic error...any help is greatly appreciated!

15.) The number of cars sold weekly by a new auto dealership grows according to a linear growth model. The first week the dealership sold two cars $\displaystyle P_0=2$ and the second week the dealer ship sold six cars $\displaystyle P_1=6$

In the first 50 weeks, how many cars did the dealership sell?

My work:

$\displaystyle S_n=\frac{(P_0+P_{n-1})(n)}{2}$

$\displaystyle P_{n-1}=P_{49-1}=P_{48}$

$\displaystyle P_{48}=P_0+nd=2+48(4)=194$

$\displaystyle S_{49}=\frac{(2+194)(49)}{2}$

$\displaystyle S_{49}=4802$

The correct answer is 5000...

17.) $\displaystyle 5+8+11+14+...+299+302=?$ (100 terms)

My work:

$\displaystyle S_{99}=\frac{(5+299)(99)}{2}=15,048$

The correct answer is 15,350

19.) $\displaystyle 15+11+7+3+...$

Find the sum of the first 100 terms

My work:

$\displaystyle P_{98}=15+(-4)(98)=-377$

$\displaystyle S_{99}=\frac{(P_0+P_{n-1})(n)}{2}=\frac{(15+(-377))(99)}{2}=-17,919$

Correct Answer: -18,300

Like I said, I'm pretty sure it's a systematic error I'm making...I've tried it lots more way besides this, but I think this is the "most" correct way I've tried....any help will be wonderful. Thanks!!