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We consider an orthogonal frequency division multiple access (OFDMA)-based Multi-access Edge Computing (MEC) system, consisting of one serving node and multiple users each with an inelastic computation task of a non-negligible task processing duration and a non-negligible computation result size. A joint uplink/downlink sub-channel, bit and time allocation problem is investigated to minimize the energy consumption, which happens to be a very challenging non-convex mixed integer nonlinear programming (MINLP) problem. We equivalently convert it into a convex MINLP problem by using the McCormick envelope, and develop two low-complexity algorithms to obtain two suboptimal solutions. Specifically, one is based on continuous relaxation with greedy rounding and the other one bases on penalty convex-concave procedure. Simulation results show the advantages of our suboptimal solutions.