In a certain class, 70% of the students scored above 90 points in the first practice, 75% of the students scored above 90 in the second practice, and 85% of the students scored in the third practice. If the score is more than 90 points, then the number of students who score more than 90 points in the three exercises accounts for at least the number of the whole class.
某班学生有70%的学生在第一次练习时的得分在90分以上，有75%的学生在第二次练习时的得分在90分以上，有85%的学生在第三次练习时得分在90分以上，那么三次练习都在90分以上学生人数至少占全班人数的多少．
<gadget id="calculator">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">(3/4) - 1</gadget>
<output>-1/4 = around -0.25</output>

<gadget id="calculator">(7/10) + (-1/4)</gadget>
<output>9/20 = around 0.45</output>

<gadget id="calculator">85 / 100</gadget>
<output>17/20 = around 0.85</output>

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

<gadget id="calculator">(9/20) + (-3/20)</gadget>
<output>3/10 = around 0.3</output>

<result>3/10 = around 0.3</result>