=

= where

B ---> speed of boat in still water

R---->speed of current

( By componendo and dividendo)

A) 180 km | B) 160 km |

C) 140 km | D) 120 km |

Explanation:

Speed in downstream = (14 + 4) km/hr = 18 km/hr;

Speed in upstream = (14 – 4) km/hr = 10 km/hr.

Let the distance between A and B be x km. Then,

x/18 + (x/2)/10 = 19 ⇔ x/18 + x/20 = 19 ⇒ x = 180 km.

A) 1/3 kmph | B) 2/3 kmph |

C) 1/4 kmph | D) 1/2 kmph |

Explanation:

Speed in upstream = Distance / Time = 3 x 60/20 = 9 km/hr.

Speed in downstream = 3 x 60/18 = 10 km/hr

Rate of current = (10-9)/2 = 1/2 km/hr.

A) 1hr | B) 1 hr 20 min |

C) 1 hr 10 min | D) 50 min |

Explanation:

Let the distance and original speed be 'd' km and 'k' kmph respectively.

d/0.8k - d/k = 20/60 => 5d/4k - d/k = 1/3

=> (5d - 4d)/4k = 1/3 => d = 4/3 k

Time taken to cover the distance at original speed

= d/k = 4/3 hours = 1 hour 20 minutes.

A) 2 9/7 km | B) 3 7/5 km |

C) 1 3/4 km | D) 4 3/8 km |

Explanation:

Relative Speed = 5 ¾ - 4 ½ = 1 ¼

Time = 3 ½ h.

Distance = 5/4 x 7/2 = 35/8 = 4 3/8 km.

A) 36 kmph | B) 35 kmph |

C) 72 kmph | D) 70 kmph |

Explanation:

By car 240 km at 60 kmph

Time taken = 240/60 = 4 hr.

By train 240 km at 60 kmph

Time taken = 400/100 = 4 hr.

By bus 240 km at 60 kmph

Time taken = 200/50 = 4 hr.

So total time = 4 + 4 + 4 = 12 hr.

and total speed = 240+400+200 = 840 km

Average speed of the whole journey = 840/12 = 70 kmph.