We have this homework..
The LRT Train Ticketing System
For each three stations I may ride on , I get to pay P15.. +5 if i exceeded more than three stations..
e.g Station1-Station5 costs P20...Station1-Station8 costs 25...
how can I display the Fee in a textbox if I put a ComboBox1 for my origin and ComboBox2 for the destination??

One way would be to handle the SelectedIndexChanged event of the second combobox. Inside the handler that you create, you can read the SelectedIndex property of each combobox to calculate the distance between stations.