Let's call the length and width of the first rectangle w + 15 and w respectively. This means that the length and width of the second rectangle is (w + 15) - 2 and w + 5 respectively. Since the perimeter of the second rectangle is 72, we can write:
The probability is just 1/2 that the disc will result with the white side landing up.
There are two sides to the disc. If they are equally likely to happen, then the chance of either side is 1/2. It does not matter which number the flip is.
