Question
John and Ivan take 4 days to assemble a toy train. Ivan and David take 5 days to build the same train while John and David take only 4 days. How many complete days would the three of them take to build the same toy train?
4 m

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John and Ivan take 4 days to assemble a toy train. Ivan and David take 5 days to build the same train while John and David take only 4 days. How many complete days would the three of them take to build the same toy train?