Do you know how to populate one combo?
If you don't i suggest that you read some tutorials before.

Ya of course I knew about populating one combo. My question here is how to populate the SECOND drop down list upon changes of selected value in the FIRST drop down list.

Well just populate second dropdown in SelectedIndexChanged event of the first dropdown.

then the page will go back to server to populate the dropdown list
i guess using AJAX is more efficient

Sure it is.
But if she does not know how to populate this way it's going to be harder for her to do it ajax way.

surely

I have four dropdownlist boxes Each contains 1,2,3.....7 subitems.
I have filled first dropdown in page_init event by using
Me.ddlPickupNoOfDays1.Items.Add("1") .......Me.ddlPickupNoOfDays1.Items.Add("7") Now I want to fill remaining 3 dropdownlist boxes with same values And I dont want to write the same code in page_init event how can I fill remaining 3 dropdowns using first If someone knows please help

Why u using this long way, why dont you edit items in dropdownlist properties, much easier, you dont have to write any code.
Hope this helps,