Let's take the data from the problem:
i = 20% = 0.2
Np = $51.44
Now, we can apply in this equation to solve for the original price (Op):
i = Np / Op
1 - 0.2 = 51.44 / Op
0.8 = 51.44 / Op
51.44 / 0.8 = Op
$64.30 = Op
The original price of the radio was $64.30.
Obs.: 0.2 was subtracted from 1 because it's a discount. If it was a increase, we just need to sum to 1.
Hope it helps.
