Your friend has a program which he builds automatically each day. He is going away for 10 days and he asks you to look at the build results each day and let him know the day the build fails. He comes back on the 11th day and his boss asks him about the build results. You forget to monitor the build results and when you check on the 11th day, the result is a fail. How would you tell which day the build failed in the least number of tries by looking at the results.
