Found another good question, this time it's not original. Comes from Physics for Scientists and Engineers with Modern Physics.
Astronauts are visiting Planet X. They take a 2.5m string with a mass of 5g and a 1kg mass tied to one end. On the planet, the astronauts fix one end of the string, horizontally extend it for 2m, hold it as shown in this picture (image1.masterfile.com/em_w/03/65/18/632-03651827em.jpg) after 2m, and allow the remaining .5 m with the mass tied to it to hang freely. The astronauts then proceed to pluck the 2m long string, finding that the string forms a standing wave at 64 Hz and at 80 Hz but at no frequencies in between. What is the gravitational acceleration on Planet X?
A useful formula: v=root(T/d) where v is the speed of a wave on a string, T is the tension in millinewtons, and d is the linear density in grams per meter.
Every great and deep difficulty bears in itself its own solution. It forces us to change our thinking in order to find it. - Niels Bohr