Question 8453
You can start by letting P = the normal selling price.
The sale price ($0.64) = the normal selling price (P) minus 20% (0.2) of the normal selling price.

So, P - 20% of P = $0.64 or, in algebrese:
  P - 0.2P = $0.64 Simplify and solve for P.
0.8P = $0.64 Divide both sides by 0.8
P = $0.64/0.8 = $0.80 or 80 cents.