There are 420 students in the fifth grade of Hongguang Primary School, and the number of students in the sixth grade is less than that in the fifth grade (1/7). How many students are there in the fifth and sixth grades?
红光小学五年级有学生420人，六年级学生人数比五年级少(1/7)，五、六年级共有多少学生？
<gadget id="calculator">1 / 7</gadget>
<output>1/7 = around 0.142857</output>

<gadget id="calculator">1 - (1/7)</gadget>
<output>6/7 = around 0.857143</output>

<gadget id="calculator">420 * (6/7)</gadget>
<output>360</output>

<gadget id="calculator">360 + 420</gadget>
<output>780</output>

<result>780</result>