What formulas would I use for the following:

A1=.30, A2=.30, Total in A3=1 (as in 1 hour)

Adding:
B1=40(minutes)
B2=6:30
Answer in B3=7:10

Subtracting:
C1=40(minutes)
C2=6:30
Answer in C3=5:50

Thanks in advance for your help!!