by Danny Roth
Published Sunday June 07 2020 (link)
George and Martha are studying optics. If you place an object a specific distance from a lens, an image will appear at a distance from that lens according the following formula:
The reciprocal of the object distance plus the reciprocal of the image distance is equal to the reciprocal of the focal length of the lens.
The object distance was a two-digit whole number of cm (ab). The image distance and the focal length of the lens were also two-digit whole numbers (cd and ef respectively). The six digits were all different and non-zero. Also, the object distance and the focal length were of the same parity and b was an exact multiple of d. Martha pointed out that the sum of those three two-digit numbers was a prime number.
What was that prime number?