Question

Raphael paid $644 for a camera during a 20% off sale. What was the​ camera's regular​ price?

Answer 1

644 is 80% of the regular price, so price=644/0.8 = 805 $

Answer 2

$S = $P - $P × 0.15 = $P(1 - 0.2) = $P × 0.8. Do a litte algebra, you get $S=.8$P. 
$S stands for sale price, $P stands for original price. We know the sales price: $644. We know now that 644=.8$P. Divide both sides by .8 and you get $P=$805. Hope this helps!