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 \(\displaystyle \frac{5}{4}\)?