The rule is when any one of the hourglasses finishes leaking, that hourglass must be flipped over immediately to keep it running.
i think the above thing needs to be taken care off...
so here is a solution
start with both the glasses
after the small hour glass finishes turn it. Time at that point will be 5 min.
after the big hour glass finishes turn it. Time at that point will be 7 min.
after small hour glass finishes turn both Big glass will have 3 min remaining and small 5 min and the time will be 10 min.
after the big hour glass finishes turn both. in turn both big will have 7 min and small will have 3 min and the time will be 13 min.
when the small gets emptied it will be 16 min.