There are 50 students in Class 51 who all participated in the fundraising activity of "Project Hope". Some of them donated 5 yuan, and the rest donated 3 yuan. The total donation was 180 yuan. Then how many students donate 5 yuan?
五一班有学生50人都参加"希望工程"的募捐活动，其中一部分同学捐款5元，其余同学捐款3元，最后统计捐款总数为180元．则捐5元的同学有多少人．
<gadget id="calculator">50 * 3</gadget>
<output>150</output>

<gadget id="calculator">180 - 150</gadget>
<output>30</output>

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<result>15</result>