Matias is a salesman in a bookstore. He sold 7 books on Tuesday and three times as many on Wednesday. If the sales from Wednesday were tripled on Thursday, how many books did Matias sell during these three days combined?
Matias sold 7 * 3 = 
<gadget id="calculator">7*3</gadget>
<output>21</output>
21 books on Wednesday, as he sold three times more books than on Tuesday.
The sales from Wednesday were tripled on Thursday, which means Matias sold then 21 * 3 = 
<gadget id="calculator">21*3</gadget>
<output>63</output>
63 books.
That means in total Matias sold 7 + 21 + 63 = 
<gadget id="calculator">7+21+63</gadget>
<output>91</output>
91 books during these three days.

<result>91</result>