Important for :

1

A

Amitabh can do a piece of work in 25 days.In 5 days he can do 1/5th work.

Rest work = 1 – | 1 | = | 4 |

5 | 5 |

Bindu can complete 4/5th of the work in 20 days so he can do the complete work in 25 days.

Amitabh's 1 day work = | 1 |

25 |

Bindu's 1 day work = | 1 |

25 |

(Amitabh and Bindu)'s 1 day work = | 1 | + | 1 | = | 2 |

25 | 25 | 25 |

Amitabh and Bindu together can complete the work in = | 25 | days |

2 |

Hence, option A is correct.

2

3

4

5

D

Let the total work be 100%A can do the work in 5 days, so he can do 20% work in 1 day

B can do the work in 4 days, so he can do 25% work in 1 day

Together they can do 20+25 = 45% work in 1 day

Now the work is increased by 800%, so the work becomes 900%

The work can be done in | 900 | = 20 days |

45 |

Hence, option D is correct.