ElectricPavlov

25000 = 12500 e^(.09 t)     take ln both sides

ln25000 = ln12500  + (.09t)

(ln25000  -  ln12500 )  /  .09  = t    =  7.702 years

I think you may have left something out of your question.....

.00084 m = .084 cm

so      .084 x 2.99 = .25116  sq cm

x, y   =  2,-9    r = sqrt(1) = 1   

x,y   = 7, -4   r = sqrt(16) = 4   

Two whole numbers multiplied together won't result in a fraction....but you CAN convert it to a fraction

Example    5 x 3 =  15    x 3/3  =  45/3

                  6 x 4  = 24    x 2/2  =  48/2

Here is a graph from the desmos calculator:

0 hrs         500 mg

8 hrs         250

16             125

24               62.5

32               31.25

40               15.625

48 hrs           7.8125 mg

Let 'x' = original cost to EACH of the three for gas....

then   3x = total gas cost

then two more join and the EACH COST goes down 11.00

so  x-11 is the new EACH COST

there are now 5 of them

5(x-11) = total cost = 3x 

5(x-11) = 3x

5x - 55 = 3x

2x = 55

x = 27.50    original cost of gas for 3    total cost = 3 (27.50) = 82.50   

WHen there are 5 the EACH COST is  82.50/5 =  16.50      which IS 11.00 than the original cost each.

There are calculators that will do this for you:                       

\(\sum_{1}^{22} x\) = 253

Basically it does this:  1 + 2 +3 + 4.....+ 22

Hence the 'averages'  and approximate..