There are 315 students in the first grade of a certain school, of which boys accounted for 20% of the total number. Later, a group of boys were transferred. At this time, boys accounted for 30% of the total number of students in the whole grade. How many boys were transferred?
某校一年级有学生315人，其中男生占总人数的20%，后来又转入一批男生，这时男生占全年级总人数的30%，又转入多少男生？
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">1 - (1/5)</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">315 * (4/5)</gadget>
<output>252</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 - (3/10)</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">252 / (7/10)</gadget>
<output>360</output>

<gadget id="calculator">360 - 315</gadget>
<output>45</output>

<result>45</result>