The school planted 500 trees in spring last year, and the survival rate was 85%. The survival rate of tree planting in autumn last year was 90%. It is known that 7 more trees were planted last autumn than in spring, and how many trees were planted last autumn.
学校去年春季植树500棵，成活率是85%，去年秋季植树成活率为90%．已知去年秋季比春季多种活了7棵树，去年秋季种了多少颗树．
<gadget id="calculator">85 / 100</gadget>
<output>17/20 = around 0.85</output>

<gadget id="calculator">1 - (17/20)</gadget>
<output>3/20 = around 0.15</output>

<gadget id="calculator">500 * (3/20)</gadget>
<output>75</output>

<gadget id="calculator">75 - 7</gadget>
<output>68</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">1 - (9/10)</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">68 / (1/10)</gadget>
<output>680</output>

<result>680</result>