The whole class subscribes to magazines. It is known that (3/4) of the whole class subscribe to "E-Times" and (4/5) of the whole class subscribe to "Window of Science and Technology". How much of the whole class is ordered by both (fill in the marks)
全班都订杂志，已知订《e时代》的占全班的(3/4)，订《科技之窗》的占全班的(4/5)．两种都订的占全班的多少(填分数)
<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) - 1</gadget>
<output>-1/5 = around -0.2</output>

<gadget id="calculator">(3/4) + (-1/5)</gadget>
<output>11/20 = around 0.55</output>

<result>11/20 = around 0.55</result>