The lectionary is structured so that the Old Testament and Gospel readings are thematically related each Sunday. The relationships for the 17th Sunday in Ordinary Time is based on persistence in prayer - and trusting that God will supply what is needed.
Too often we act on the subconscious belief that "if a little is good, more is better." Then we find that no amount is enough. We need only look at celebrities to see where this leads. Yet is not just celebrities who have this problem. We all know people who can't get enough work, food, possessions, time looking at facebook, knowledge, etc.
Gregory Pierce, in his book Spirituality at Work: 10 Ways to Balance Your Life on the Job sets forth ten disciplines, Number nine is "Deciding how much is enough - and sticking to it." (Go here for my review.) Setting goals is good. Reaching them is also good. There does, however, come a time when we have enough and wishing for more becomes self destructive.
So, how does this relate to the readings? In Gen. 18:20-32 We find Abraham asking the Lord how many righteous ones he must find in order to save Sodom. He negotiates until he reaches the number of ten righteous men. These will be enough.
In Luke 11:1-13 we are told to pray for our daily bread. We need not fear that we will not have enough for tomorrow. We don’t have to pray for twice as much bread as we will need today. We can ask for that tomorrow. We should not be afraid to consume what we have today, fearing that we will starve tomorrow.
Are we sure we won't get enough of the right things for tomorrow? On a human level, we can never be absolutely certain - but we can turn our lives over to the care of God today. That will be enough for today. We can do the same tomorrow.