Someone

alias task {
  if (%amount != $null) {
    /echo -a You already have an ongoing slayer task of %amount %task $+ 's! Type /done to end it.
  }
  else {
    set %amount $1
    set %hp $2
    set %task $3-
    /msg $active Slayer Task: %amount %task $+ 's. That's $calc( %hp * %amount ) Slayer, $calc( %hp * %amount * 1.3) HP and $calc( %hp * %amount * 4) combat experience!
  }
}

alias done {
  if (%amount == $null) {
    /echo -a You do not have any active slayer task, type /task amount hp task to start one!
  }
  else {
    /msg $active My task of %amount %task $+ 's is complete! I gained $calc( %hp * %amount ) Slayer, $calc( %hp * %amount * 1.3) HP and $calc( %hp * %amount * 4) combat experience!
    unset %task
    unset %amount
    unset %hp
  }
}
alias burth {
  if (%amount == $null) {
    /echo -a You do not have any active slayer task, type /task amount hp task to start one!
  }
  else {
    /msg $active My task of %amount %task $+ 's has been burthed.
    unset %task
    unset %amount
    unset %hp
  }
}