If two circles of radii r1 and r2 touch internally, the distance between their centres is equal to
r1+ r2
2r1
2r2
r1 - r2
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A, B and C can do a work in 12, 15 and 20 days respectively. They all work for one day. Then B leaves the group. In how many days will A and C completethe rest of the work?