title: R: calculate just with times (to find for example a pace per km)
body: <p>How divide or add times? I have a time in a race of 10 km and I want to find the pace per km for each runner from the result of the race.
  For example, if I have a <code>data.frame</code> with a column <code>time</code> as <code>00:50:00</code> in a row (for example) for a run of 10 km how can I find the pace for 1 km ("00:50:00" / 10 km). With <code>strptime</code> I transform the string in <code>H:M:S</code> but when I will divide the column by 10 to find the pace in one km, R answers <code>Erreur dans Ops.POSIXt(d$temps, 10) :  '/' non défini pour des objets "POSIXt</code>. And I have unexpected date in time with <code>strptime</code>.
  Thanks</p>