Example:
Processes p0, p1, and p2 compete for 12 tape drives
max need current usage could ask for
p0 10 5 5
p1 4 2 2
p2 9 2 7
3 drives remain
current state is safe because a safe sequence exists: <p1,p0,p2>
p1 can complete with current resources
p0 can complete with current+p1
p2 can complete with current +p1+p0
if p2 requests 1 drive, then it must wait because that state would be unsafe.