\(\Delta \nu=\nu_{m+1}-\nu_m=\frac{(m+1)c}{2d}-\frac{mc}{2d}=\frac{c}{2d}=\frac{3*10^8m/s}{2(1m)}=0.15GHz\)

the number of standing wave modes=\(\frac{6GHz}{0.15GHz}=40\)