Question

Which would result in a lower price to first discount an item by 10% and then by a further 15%, OR to first discount an item by 15% and then by a further 10%. Justify your reasoning.

Answer 1

Neither one. They will both result in the same price.

Explanation:

To discount an item 10%, you charge 90% of the price of the item. To find 90% of a price, you multiply the price by 0.9.

To discount an item 15%, you charge 85% of the price of the item. To find 85% of a price, you multiply the price by 0.85.

Since multiplication is commutative, multiplying a number by 0.9 and then by 0.85 is the same as multiplying the number by 0.85 first and then by 0.9.

Let's say the item costs x.

Take off the 10% discount first: 0.9x

Now take off the 15% discount: 0.85 * (0.9x)

Now do it the other way.

Take off the 15% discount first: 0.85x

Now take off the 10% discount: 0.9 * (0.85x)

Since 0.85 * 0.9 * x = 0.9 * 0.85 * x, the results are the same.

Answer: neither