An engine has an efficiency e1. The engine takes input heat of magnitude |QH| from a hot reservoir and delivers work of magnitude |W1|. The heat rejected by this engine is used as input heat for a second engine, which has an efficiency e2 and delivers work of magnitude |W2|. The overall efficiency of this two-engine device is the magnitude of the total work delivered (|W1|) + |W2|) divided by the magnitude |QH| of the input heat. Find an expression for the overall efficiency e in terms of e1 and e2.