A classmate needs 9 exercise books. There are 42 students in Class 4 1 and 39 students in Class 4 2. How many exercise books do these two classes need? How would you count?
一个同学要用9本练习本，四1班有42人、四2班有39人，这两个班共需要多少本练习本？你会怎样算？
<gadget id="calculator">42 * 9</gadget>
<output>378</output>

<gadget id="calculator">39 * 9</gadget>
<output>351</output>

<gadget id="calculator">378 + 351</gadget>
<output>729</output>

<result>729</result>