Exactly which part do you not get?  First, Do you understand what it means to "reduce a price by 20%"?   If the original price was $30 what is 20% of that?  What is the price after you take away that 20%?  Do you understand that if you "reduce" a price by 20% you are leaving 100- 20= 80%?  Do you understand that "80%" means 80/100?  So reducing a price by 20% the new price is 80% of the original price?  That is how Jomo got "\(\displaystyle \left(\frac{80}{100}\right)p= 8\)" where "p" is, as Jomo said, the unknown old price and "8" is the given new price.
Or is your problem solving that equation?  Do you know that \(\displaystyle \frac{80}{100}= \frac{8}{10}= \frac{4}{5}\)? So you were asked to solve the equation \(\displaystyle \frac{4}{5}p= 8\).  What do you get if you multiply both sides of the equation by \(\displaystyle \frac{5}{4}\)?