ואתחנן אל ה’ בעת ההוא לאמר (ואתחנן ג:כג)

The Medrash says that Moshe Rabbeinu davened 515 tefillos, the numerical value of the word ואתחנן.  The numerical value of the word תפילה is also 515.  We learn from this that a tefillah can be considered a prayer when it comes to a request.  The Abudram adds that the words ‘עבודת לב’ – work of the heart, and ‘בכונת הלב’ – one’s thoughts and intentions, the numerical value is also 515 – to teach us that our prayers need be with כונה!