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In this paper, we consider cooperative communication by secondary users in a multicast primary network. Secondary users act as relays for primary network and are allowed to access primary channel during a part of the transmission time. We consider that the secondary system pays incentive to encourage the primary system in transmission time sharing. We formulate an optimization problem to find the time allocation between the primary and the secondary transmission to maximize the primary utility while keeping the secondary utility above certain level. However, the optimal time allocation problem turns out to be intractable. We simplify the optimal time allocation problem by approximation. The simplified problem falls into a linear programming and is easily solved. Numerical examples are presented to give insight of optimal time sharing and to unveil the impact of system characteristics on.