如果a＞0，b＜0，且a、b两数的和为正数，那么（　　）

   Bob was cutting a branch（树枝） off a tree in his garden. While he was sawing（锯）, another man passed in the street. He stopped and said, “ Excuse me, but if you continue (继续)to saw（锯） that branch like that, you will fall down with it.” He said this because Bob was sitting on the branch and cutting it at a place between himself and the trunk（树干） of the tree.
   Bob said nothing. He thought, “ This is some foolish（愚笨的） person who has no work to do and goes about telling other people what to do and what not to do.”
   The man continued on his way. Of course, after a few minutes. The branch fell and Bob fell with it.
   “My God!” he cried. “That man knows the future!” and he ran after him to ask how long he was going to live. But the man had gone.

一把杆秤不计自重，提纽到秤钩距离是3cm，秤砣质量250g。用来称质量是2kg的物体，秤砣应离提纽多远，杆秤才平衡？若称量某物体质量平衡时，秤砣距提纽60cm，该物体质量多大？