  We investigate the ratio of the branching fractions of the molecular
candidates decaying into the ground and radially excited states within the
quark interchange model. Our numerical results suggest that these molecular
candidates are more likely to decay into the radially excited states than into
ground states. Especially, the ratio $\Gamma[Z_c(4430)\to
\pi\psi(2S)]$/$\Gamma[Z_c(4430)\to \pi J/\psi]\sim 9.8$ is close to the
experimental measurement, which supports the interpretation of $Z_c(4430)$ as
the $\bar{D}D^*(2S)$ molecular state. The ratios of the branching fractions of
$Z_b(10610)$ and $Z_b(10650)$ to $\pi\Upsilon(2S, 3S)$ and $\pi\Upsilon(1S)$
agrees very well with Belle's measurement. We also predict the similar ratios
for $Z_c(3900)$, $Z_c(4020)$, $R_{Z_c(3900)}$$\approx$1.3 and
$R_{Z_c(4020)}$$\approx$$4.7$. Hopefully the $\pi\psi(2S)$ mode, and ratios
$R_{Z_c(3900)}$ and $R_{Z_c(4020)}$ will be measured by the BESIII and Belle
collaborations in the near future, which shall be very helpful to understand
the underlying dynamics of these exotic states.
