Time and Work

9.A man travelled a distance of 61 km in 9 hours. He travelled partly on foot at 4 km/hr and partly on bicycle at 9 km/hr. What is the distance travelled on foot? A)15km B)14km C)16km D)17km Answer : Option C Explanation: Let the time in which he travelled on foot=x hr Then the time in which he travelled on bicycle =(9−x) hr distance = speed Ũ time ⇒4x+9(9−x)=61 ⇒4x+81−9x=61 ⇒5x=20 ⇒x=4 Distance travelled on foot =>4x=4*4=16km 10.A train covers a distance in 50 min, if it runs at a speed of 36 kmph on an average. The speed at which the train must run to reduce the time of journey to 30 min will be. A)20 kmph B)30 kmph C)40 kmph D)15 kmph Answer : Option D Explanation: Time = 50/60=5/6 hr Speed = 36 mph Distance = SŨT=36Ũ5/6=30 km Time = 30/60 hr New speed = 30Ũ1/2=15 kmph 11.A man can row 4.5 km/hr in still water and he finds that it takes him twice as long to row up as to row down the river. Find the rate of the stream. A)1.5 km/hr B)1.4 km/hr C)1.6 km/hr D)1.7 km/hr Answer : Option A Explanation: Let the speed of the current be xx km/hr Thus upward speed = (4.5−x)(4.5−x) km/hr and downward speed = (4.5+x)(4.5+x) km/hr Let distance travelled be yy, then for the same distance yy, Time Rowing UpwardsTime Rowing Upwards=2ŨTime Rowing Downwards=2ŨTime Rowing Downwards (y/4.5−x)=2*(Ũ/4.5+x) ⇒ x=x= 1.5 km/hr 12.A motorboat whose speed is 15 km/hr in still water goes 30km downstream and comes back in four and a half hours. The speed of the stream is: A)4 km/hr B)6 km/hr C)5 km/hr D)7 km/hr Answer : Option C Explanation: Let the speed of the stream be 'ss' km/hr. Then, upward speed = (15−s)(15−s) km/hr and downward speed = (15+s)(15+s) km/hr Therefore, 30/(15+s)+30/(15−s)=4.5 On solving this equation we get, s = 5 km/hr

« 1 2 3 »