не удалось установить сервер NTP, потому что ibopts25 недоступен [закрыто]

У меня ubuntu server 15.04, и теперь я попытался установить NTP-сервер, но не могу установить, потому что не было некоторых пакетов sudo apt-get install

root@server:/home/technical# apt-get install ntp
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
  libopts25
Suggested packages:
  ntp-doc
The following NEW packages will be installed:
  libopts25 ntp
0 upgraded, 2 newly installed, 0 to remove and 1 not upgraded.
Need to get 481 kB of archives.
After this operation, 1,698 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
WARNING: The following packages cannot be authenticated!
  libopts25 ntp
Install these packages without verification? [y/N] y
Err http://id.archive.ubuntu.com/ubuntu/ vivid/main libopts25 amd64   1:5.18.4-3
  404  Not Found [IP: 91.189.88.161 80]
Err http://id.archive.ubuntu.com/ubuntu/ vivid-updates/main ntp amd64 1:4.2.6.p5+dfsg-3ubuntu6.3
  404  Not Found [IP: 91.189.88.161 80]
E: Failed to fetch http://id.archive.ubuntu.com/ubuntu/pool/main/a/autogen/libopts25_5.18.4-3_amd64.deb  404  Not Found [IP: 91.189.88.161 80]

E: Failed to fetch http://id.archive.ubuntu.com/ubuntu/pool/main/n/ntp/ntp_4.2.6.p5+dfsg-3ubuntu6.3_amd64.deb  404  Not Found [IP: 91.189.88.161 80]

E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

кто-нибудь знает, как это исправить?