A project can be completed in 10 days and B in 20 days, and can also be completed in 15 days and B in 12 days. Now B will do it for 4 days first, how many days will A need to finish it?
一项工程，甲做10天乙20天完成，甲15天乙12天也能完成．现乙先做4天，问甲还要多少天完成？
<gadget id="calculator">5 / 8</gadget>
<output>5/8 = around 0.625</output>

<gadget id="calculator">12 * (5/8)</gadget>
<output>15/2 = around 7.5</output>

<gadget id="calculator">15 + (15/2)</gadget>
<output>45/2 = around 22.5</output>

<gadget id="calculator">4 * (5/8)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">(45/2) - (5/2)</gadget>
<output>20</output>

<result>20</result>