The number of students in the third grade is 75% of that in the fourth grade, and there are 120 students in the fourth grade. How many fewer students are in the third grade than in the fourth grade?
三年级人数是四年级的75%，四年级有120人．三年级比四年级少多少人？
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">120 * (3/4)</gadget>
<output>90</output>

<gadget id="calculator">120 - 90</gadget>
<output>30</output>

<result>30</result>