1. Amit goes to his office by car at the speed of 80 km\hr and reaches 15 minutes earlier. If he goes at the speed 60 km/hr, he reaches 15 minutes late. What will be the speed (in km/hr) of the car to reach on time ?
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By: anil on 05 May 2019 03.04 pm
Let ideal time taken to reach on time = $$t$$ hours Speed is inversely proportional to time => $$frac{80}{60}=frac{t+frac{1}{4}}{t-frac{1}{4}}$$ => $$80t-20=60t+15$$ => $$80t-60t=20t=15+20$$ => $$t=frac{35}{20}=frac{7}{4}$$ hours Thus, distance covered by going at 60 km/hr and reaching in $$(frac{7}{4}+frac{1}{4}=2)$$ hours = $$60 imes2=120$$ km $$ herefore$$ Ideal speed to reach on time = $$frac{120 imes4}{7}=68frac{4}{7}$$ km/hr => Ans - (C)
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