There are at least a few ways to do it, one of them being:
$8 - $5 = $3, so we may want to find out how much percent of 5 is 3.
3 = x% of 5
3 = x/100 * 5
3 = x/20 * 1
3 = x/20 / * 20 (both sides)
x = 60 [percent]
So now we know that 3 is 60% of 5. It also means that the increase from $5 to $8 was by 60%.
