Here's my take: E. OA? z = retail price y = discount price x = y - .1y (the additional 10% discount) SOLVE FOR .1y. 1) y = .9z Insuf because we have no info about dollar amounts. 2) y + 20 = x - 14 Insuf because we can't solve for any static dollar amounts. y could be 80 and x could be 114. etc. Co...