She needs to sell $4,000 worth of merchandise.
If she needs to earn $320, and she earns 8% commission, you’re looking for what 8% of original amount equals $320.
X = original amount
8% = .08
$320 = amount needed
X * .08 = $320.
X = $320/.08
X= $4,000
Kristen needs to sell $4,000 worth of merchandise at an 8% commission rate to earn $320.