When the number in □ is how many, (7/15)+(2/7)+□ can use the operation law to make the calculation simple.
当□里的数是多少时，(7/15)+(2/7)+□能够运用运算定律使计算简便．
<gadget id="calculator">8 / 15</gadget>
<output>8/15 = around 0.533333</output>

<result>8/15 = around 0.533333</result>