We know that, say, 60 hours is too many. And 10 hours is too little. So there must be a most efficient number in between; why would it be a round number like 40

We know that, say, 60 hours is too many. And 10 hours is too little. So there must be a most efficient number in between; why would it be a round number like 40, the exact number we happened to decide on decades ago? Science sounds like the best way to find that number.