We can start the solution by stating speed in feet per second (use speed = distance/time). That would be
$$\frac{230}{40}$$ feet/second
We then convert it to feet/hour. To do this we multiply the numerator by 60. The reason behind this is that a person can travel 60 times the distance in an hour as compared to 1 second at the same speed.
So we get : $$\frac{230\cdot60}{40}$$ feet/hour
Now the final step is to convert to miles/hour. As 1 mile = 5280 feet we know a person can travel 1/5280 times the distance in feet at the same speed in the same given time. So we simply divide our previous answer by 5280. So our final answer is:
$$\frac{230\cdot60}{40\cdot5280}$$ miles/hour