Carry Over Minutes Only

I am using Microsoft Excel 2003. My question is about calculating time. 1 hour + 1 hour and fifteen minutes would equal two hours and fifteen minutes. Using Microsoft Excel 2003, let's say I am using cells A1, A2, A3 and A4.

A1 will be 1:00 for 1 hour
A2 will be 1:15 for 1 hour and fifteen minutes
A3 will be my total for adding cells A1 and A2 and the answer will be 2:15 for two hours and fifteen minutes.

My specific questions is: Would it be possible for me to have the fifteen minutes (0:15) from the two hours and fifteen minutes (2:15) automatically carry over to cell A4 or cell A4 of another worksheet without having to type in 0:15 or having 2:15 appearing in cell A4?