Two strings with linear densities of 0.005kg/m are stretched over pulleys, adjusted to have vibrating lengths of 0.50m, and attached to hanging blocks. The block attached to string one has a mass of 20kg and the block attached to string two has a mass of M. When driven at the same frequency; the two strings support the standing waves as shown above(If can't see picture, string one has mode of 1 and string two has mode of 3). The frequency is 198 Hz.
What is the mass of the block suspended from string two?