Emily learnt how to ride a horse recently. Just to test her skills, she rode the horse from her home till a mine which she covered in three minutes and returned back home from the same path only this time she took four minutes. The reason was that while riding away from home, she was supported by wind however while riding back, she was riding against the wind.
How fast do you think she could have rode a mile if there was no wind ?
3 minutes and 25 5/7 seconds.
The most common way how most of people see this problem is adding 3 and four to obtain 7 and they believe it should give a correct average and thus her time should be taken to 3 and half minutes. But if you do like that, you will get to a wrong answer. This is because, the wind has only helped her for three minutes while it has worked against her for four minutes.
If she could ride a mile in three minutes with the wind, it is evident that she could go a mile and 1/3 of a mile in four minutes. Thus two and 1/3 miles in eight minutes will give her actual speed as the wind has helped her as much as it has worked against her.
Thus her actual time for a single mile without wind will be 3 minutes and 25 5/7 seconds.