N = (\frac{N_{1}}{v_{1}} + \frac{N_{2}}{v_{2}}) v \Rightarrow v = \frac{(N_{1} + N_{2}) v_{1} v_{2}}{N_{1} v_{1} + N_{2} v_{2}} = 12, 5 m / s

${N=\left({N_1 \over v_1} + {N_2 \over v_2}\right) v \quad \Rightarrow \quad v = \frac{(N_1 + N_2) v_1v_2}{N_1v_1+N_2v_2}={12,5}\,\mathrm{m/s}}$

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