There is a circular fountain in the campus of the Experimental Primary School, which covers an area of 78.5m**2. Now it is necessary to expand the fountain. How much?
实验小学校园内有一个圆形喷水池，占地面积为78.5m**2，现在要对这个喷水池进行扩建，计划扩建后喷水池的占地面积为153.86m**2，问扩建时喷水池的直径应增加多少？
<gadget id="calculator">7 - 5</gadget>
<output>2</output>

<gadget id="calculator">2 * 2</gadget>
<output>4</output>

<result>4</result>