Four runners take turns to run around a 400 m rectangular race course such that one runner runs across one side only. They run with speeds of 'a' m/s, 'b' m/s, 'c' m/s and 'd' m/s, though not necessarily in that order. The values of a, b, c and d are four consecutive numbers such that only one number among a, b, c and d is a composite number. What is the maximum time (Tmax) in which the four runners can cover the whole distance (Tmax is an integer)?
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