During Arbor Day, the students went to plant trees. They planted 7 rows, 9 trees in each row, but 4 died. How many survived?
植树节时同学们去种树，种了7行，每行9棵，结果死了4棵，成活了多少棵？
<gadget id="calculator">9 * 7</gadget>
<output>63</output>

<gadget id="calculator">63 - 4</gadget>
<output>59</output>

<result>59</result>