During a 2 year inflation period, a car's price increased from 18000 to 25920. what was the percent inflation per year?

You have done it simple interest Mathematician. Compound rate is probably more appropriate.

 

$$FV=PV(1+r)^n\\
25920=18000(1+r)^2\\
25920/18000=(1+r)^2\\
\sqrt{25920/18000}=1+r\\
r=\sqrt{25920/18000}\;-1\\$$

 

$${\sqrt{{\frac{{\mathtt{25\,920}}}{{\mathtt{18\,000}}}}}}{\mathtt{\,-\,}}{\mathtt{1}} = {\frac{{\mathtt{1}}}{{\mathtt{5}}}} = {\mathtt{0.2}}$$

 

0.2*100 = 20%

First we have to find out how much the car's price increased by. So we subtract the original value from the increased value: 25,920 - 18,000 = 7,920. Next we divide 7,920 by 2, since it's a two year period and the question is asking for one year: 7,920 ÷ 2 = 3,960. Now that we know how much it increased (3,960), we put that in a fraction over our original value (18,000) to find out the actual rate of increase:

$${\frac{{\mathtt{3\,960}}}{{\mathtt{18\,000}}}} = {\frac{{\mathtt{11}}}{{\mathtt{50}}}} = {\mathtt{0.22}}$$   Multiplying 0.22 by one hundred, we get 22%.