A family made a $$100\text{km}$$ trip. For half the distance, they drove at $$50\text{km}$$~ per hour; for the other half, they drove at $$25\text{km}$$~ per hour How many hours did this trip take? 
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For the first $$50\text{km}$$, they drove at $$50\text{km/hr}$$-that took $$1\text{hr}$$. For the other second $$50\text{km}$$, they drove at $$25\text{km/hr}$$-that took $$2\text{hrs}$$ . The trip took $$3\text{hrs}$$ . 