At Deoprayag (Garhwal, UP) river Alaknanda mixes with the river Bhagirathi and

At Deoprayag (Garhwal, UP) river Alaknanda mixes with the river Bhagirathi and becomes river Ganga. Suppose Alaknanda has a width of $12 \mathrm{~m}$, Bhagirathi has a width of $8 \mathrm{~m}$ and Ganga has a width of $16 \mathrm{~m}$. Assume that the depth of water is same in the three rivers. Let the average speed of water in Alaknanda be $20 \mathrm{~km} / \mathrm{h}$ and in Bhagirathi be $16 \mathrm{~km} / \mathrm{h}$. Find the average speed of water in the river Ganga.