A) 45 km/h
B) 55 km/h
C) 60 km/h
D) 65 km/h
Correct Answer: B
Solution :
Original speed of car = x km/h, then \[\frac{715}{(x+10)}=\frac{715}{x}-2=\frac{715-2x}{x}\] \[\Rightarrow \] \[715x=(715-2x)(x+10)\] \[\Rightarrow \] \[715x=715x-2{{x}^{2}}+7150-20x\] \[\Rightarrow \] \[2{{x}^{2}}+20x-7150=0\] \[\Rightarrow \] \[{{x}^{2}}+10x-3575=0\] \[\Rightarrow \] \[(x-55)(x+65)=0\] \[\therefore \,\] \[x=55km/h\]
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