Confused... Please Help

Let the original price   = P

And 20% off  the original price   = .20P

So

Original Price -  [ 20% off ]  =  640     ...  so..

P  - .20P  = 640       simplify

.80P  =  640        divide both sides by  .80

P = 640 / .80  = 800

So.....the original price was $800

The original price is $800.00

Let x = phone's original price

Let 0.2 (20%) = discount of phone

Let 640 = price of phone after discount

$0.2x=x-640$

$0.2x-x=x-640-x$

$-0.8x=x-640-x$

$-0.8x=-640-0$

$-0.8x=-640$

$\frac{-0.8x}{-0.8}=\frac{-640}{-0.8}$

$1x=\frac{-640}{-0.8}$

$x=\frac{-640}{-0.8}$

$x=800$