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15a. Okay. So each of the two bikes cost $380. One of them has a 50% discount, and the other is 30% off, with another 20% off later. For the first one, all we have to do is multiply by 50% (0.5). For the second one, what we do is multiply by 70%, and then by 80%, because while you get a discount, you still pay a portion of those prices. Keep in mind that 70% (0.7) is multiply by 380, and then the remainder of that number is multiplied by 80% (0.8). If you multiply correctly your answers should be this:

First bike: $190
Second bike: $212.80

15b. Eliza should by the first bike, because the cost of that bike is cheaper. When you subtract the prices, you will see that she will pay $22.80 less if she buys the first bike, compared to the second bike. 

16a. Okay. So for this question, the price is marked up by 50%. To find the price that the shoes will be sold, we will multiply by 150%. This is because adding 100% to the markup will help us get straight to the selling price. 57 * 150% (1.5) is 85.5. There. The selling price of the shoes is $85.50.