A cylindrical oil barrel is filled with oil, and the oil in the barrel is poured out (2/5), and there are 15 liters left. It is known that the height of the oil barrel is 5 decimeters, what is the area of the bottom of the oil barrel in square decimeters?
一个圆柱形的油桶，里面装满油，把桶里的油倒出(2/5)，还剩15升．已知油桶的高是5分米，油桶的底面积是多少平方分米？
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 - (2/5)</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">5 * (3/5)</gadget>
<output>3</output>

<gadget id="calculator">15 / 3</gadget>
<output>5</output>

<result>5</result>