$$v_S-v_R =\frac{35km}{205 min} \cdot 60 \frac{min}{h}$$
$$v_S+v_R =\frac{35km}{75 min} \cdot 60 \frac{min}{h}$$
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$$v_S =\frac{35km}{205 min} \cdot 60 \frac{min}{h} +v_R$$
$$v_S =\frac{35km}{75 min} \cdot 60 \frac{min}{h}- v_R$$
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$$v_S =v_S$$
$$\frac{35km}{205 min} \cdot 60 \frac{min}{h} +v_R =\frac{35km}{75 min} \cdot 60 \frac{min}{h}- v_R$$
$$\frac{35km}{205 min} \cdot 60 \frac{min}{h} +2 v_R =\frac{35km}{75 min} \cdot 60 \frac{min}{h}$$
$$2 v_R =\frac{35km}{75 min} \cdot 60 \frac{min}{h} - \frac{35km}{205 min} \cdot 60 \frac{min}{h} $$
$$2 v_R =(\frac{1}{75 min} - \frac{1}{205 min} )\cdot 35km \cdot 60 \frac{min}{h} $$
$$v_R =(\frac{1}{75} - \frac{1}{205 } )\cdot \frac {35 \cdot 60 }{2}\frac{km}{h} $$
$$v_R =8,878\frac{km}{h} $$