Mass-balance (mixing) calculation for a dissolved conservative tracer (Sr) in a river network: the concentration at a downstream point equals the weighted average of contributing tributary concentrations by their water mass fractions.
At point A, water consists of contributions from R (mass fraction = 1 - 0.2 - 0.2 = 0.6), T1 (0.2), and T2 (0.2). So...
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