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Two fundamental functions of the sensor nodes in a wireless sensor network are to sense its environment and to transmit sensed information to a basestation. Heterogeneous wireless sensor networks are composed of a large number of sensors equipped with different transmission and sensing capabilities. The basestations are more powerful than sensors. In this paper, we study a relay node placement problem, which aims to deploy a minimum number of relay nodes to establish directed paths from any sensor node to a basestation, in heterogeneous wireless sensor networks with basestations. For this problem, we present a polynomial time 6-approximation algorithm and give an example which shows that 5 is a lower bound of the approximation ratio of the algorithm.