A, B, and C have to complete a set of exercises. The number of questions completed by A and B accounts for (2/5) of the total number of questions, and the number of questions completed by B and C accounts for (9/5) of the total number of questions. 10), what fraction of this exercise set did B complete?
甲、乙、丙三人要做完一本习题集，甲、乙做完的题数占总题数的(2/5)，乙、丙做完的题数占总题数的(9/10)，乙完成这本习题集的几分之几？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

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

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

<gadget id="calculator">(2/5) + (-1/10)</gadget>
<output>3/10 = around 0.3</output>

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