Given the task definition and input, reply with output. You are given a math word problem and you are supposed to apply a single mathematical operator like addition, subtraction, multiplication, or division on the numbers embedded in the text to answer the following question and then only report the final numerical answer.

her last stop was the post office . there she sent her rock collection to the geological museum in london . she was left with her 15 favorite stones after sending away a part of her collection . if she originally has 78 stones in her collection how many did she send away ?
63