Let the lowest frequency be f. The frequencies form an arithmetic progression: f, f+4, f+8, ... . The 10th term is $f_{10} = f + (10-1) \times 4 = f + 36$. Given $f_{10} = 2f$. So, $2f = f + 36$, which means f = 36 Hz. The highest frequency is 2f = 72 Hz.