A, B, C, and D are arranged in a straight line like this: B is 8 cm away from A; C is 15 cm away from A; the sum of the distances between D and B and D and C is 10 cm, find the distance between D and A in centimeters.
A、B、C、D在一条直线上如此排列：B距A，8厘米；C距A，15厘米；，D距B与D距C的距离和是10厘米，求D距A多少厘米．
<gadget id="calculator">10 - 7</gadget>
<output>3</output>

<gadget id="calculator">3 / 2</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">8 - (3/2)</gadget>
<output>13/2 = around 6.5</output>

<result>13/2 = around 6.5</result>