There is a joke saying that when I graduated from high school, I found my old man knew nothing. When I graduated from college, I found he learned a lot during the past four years. Now I am the old man, and found I really knew less than when my son graduated from high school, or even when I graduated from high school. I cannot use the algebra that learned at Pui Ching to solve simply daily real world problems and had to resort to my grade school arithmetic. May be that was the reason I was in section A at Pui Ching.

Every morning I walk to the Star Ferry, take the Ferry to Central, and then walk to the office. Since my office overlooks the Ferry, when I take a break, I look out the windows watching the Ferryboats sailing back and forth. 

I can see during the rush hours, there are four boats sailing back and forth, and three during rest of the time. I read the notice posted at the dock saying it take 7 min. to sail across the harbor. I timed and find out each boat will stay at the dock for approximately 3 minutes for loading and unloading the passengers. 

Now what is the average time it will take one to across the harbor during the rush hours (when there are 4 boats), and during the non-rush hour (3 boats)? How do you write an algebraic equation to solve this? For those from sections C & D studied calculus at Pui Ching, if you wish to apply that to solve this problem, I am eager to learn.