CSS3 calc()

#a {
  width:75%;
  margin-right: 1em;
}
#b {
  width: -moz-calc(25% - 1em);
}

input {
  padding:2px;
  border:1px solid black;
  display:block;
  width: -moz-calc(100% - 2 * 3px);
}

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