We have two (narrow) capillary tubes $T_1$ and $T_2$. Their lengths are $l_1$ and $l_2$ and radii of cross-section are $r_1$ and $r_2$ respectively. The rate of flow of water under a pressure difference $P$ through tube $T_1$ is $8cm^3/sec$. If $l_1 = 2l_2$ and $r_1 =r_2$, what will be the rate of flow when the two tubes are connected in series and pressure difference across the combination is same as before ($= P$)