1. **State the problem:** We need to find the cost price of a product that retails for 399 with a gross profit (GP) of 30% of the retail price. 2. **Formula used:** Gross Profit (GP) is given by $$GP = Retail\ Price - Cost\ Price$$ Since GP is 30% of the retail price, we write: $$GP = 0.30 \times Retail\ Price$$ 3. **Express cost price:** $$Cost\ Price = Retail\ Price - GP$$ 4. **Calculate GP:** $$GP = 0.30 \times 399 = 119.7$$ 5. **Calculate cost price:** $$Cost\ Price = 399 - 119.7 = 279.3$$ **Answer:** The cost price is 279.3.