A movie theater sold a total of 1,050 front and rear row movie tickets, and received a total of 3,900 yuan. Each ticket in the back row was 3.5 yuan, and each ticket in the front row was 4 yuan. How many tickets are there in the front row?
某电影院共售出前后排电影票1050张，共收款3900元，后排每张3.5元，前排每张4元，问前排票有多少张．
<gadget id="calculator">3.5 * 1_050</gadget>
<output>3_675</output>

<gadget id="calculator">3_900 - 3_675</gadget>
<output>225</output>

<gadget id="calculator">4 - 3.5</gadget>
<output>0.5</output>

<gadget id="calculator">225 / 0.5</gadget>
<output>450</output>

<result>450</result>