Percentage Unchanged

If the base of a rectangle is increased by 10% and the area is unchanged, by what percentage (exactly) is the width decreased by ?

CD Heaven

All CD Heaven stores were given the same number of a popular CD to sell for £24. In their two week sale each store reduces the price of the CD by 25% ... How many CDs did the store sell at each price?

A Change in Code

There are two sets of numbers. The second is the result of the first after an increase by a constant percentage. How can you find that percentage if one set of numbers is in code?

Percentage Swap

Stage: 4 Short Challenge Level:

$2007\%$ of $50$ is $2007\times50/100$, which is the same as $50\times2007/100$, or $50\%$ of $2007$. Hence the answer is $100\%$ of $2007$, which is $2007$.

This problem is taken from the UKMT Mathematical Challenges.