Praveen's Blog

An Eternal Quest for Incremental Improvement

Puzzle of the week - Week #50

You are driving by car to a particular destination, and our only assumption is that you are free to drive at any speed you choose - no traffic jams or anything like that. For the first half of the journey (and by half we mean half the overall distance between the starting point and your finishing point) you drive at 20 miles per hour. You then realise that this is all taking much too long, and that you are going to be late. You therefore decide that you will increase your speed so that your overall average speed for the whole journey will be 40 miles per hour. How fast do you have to drive for the remaining part of your journey in order for your average speed for the whole journey to be 40 miles per hour? Strangely enough the answer is the same, whatever the total distance of your journey.