Let's use some numbers. Fix the original price of the boat at 100. Then, the dealer bought it at (1-0.10)*100=90 with the discount of 10%. Then, the profit is 30% of 90, i.e. 0.3*90=27. Thus, the selling price must be 90+27=117, given the formula profit = sell price - cost. Now, keeping the selling price at 117, if the dealer would have paid the boat fully, the cost would have been 100 and the profits 117-100=17. Then, the profits are 17/100*100% of the cost, i.e. 17%.