A new book sold 15 copies at the bookstore on Wednesday. On Thursday the book sold three times that many copies. On Friday the book sold one-fifth the number of copies it did on Thursday. How many copies of the book did the bookstore sell by Friday?
On Thursday 15*3=
<gadget id="calculator">15*3</gadget>
<output>45</output>
45 books were sold.
On Friday 45/5=
<gadget id="calculator">45/5</gadget>
<output>9</output>
9 books were sold.
Adding up all three days 15+45+9=
<gadget id="calculator">15+45+9</gadget>
<output>69</output>
69 books were sold by Friday.

<result>69</result>