I guess **4 paris are possible **

We know that hcf is higest common factor in both numbers which is 20 here. Both numbers will be multiple of 20.

remaining factors of the numbers is 800/20 = 40

we just need to distribute the prime factors (2*2*2*5) of 40 to both numbers with 20.

supoose both numebrs are A and B

A |
B |

2 *20 =40 |
2*2*5*20 =400 |

5*20=100 |
2*2*2*20 =160 |

2*2*20=80 |
2*5*20=200 |

2*2*2*5*20 =800 |
1*20=20 |

A*B =LCM * HCF = 800*20 = 16000