下表显示了参加数学竞赛的 120 名学生的成绩。A 等是最好成绩，E 等是最差成绩。

成绩等第	A	B	C	D	E
学生人数	15	27	54	18	6

只有 $\frac{7}{20}$ 的学生进入了第二轮。
进入第二轮所需的最低成绩等第是多少？

1. Calculate the number of students who made it to the second round.

$\frac{7}{20} \times 120 = 7 \times 6 = 42$

2. Determine the minimum grade by accumulating students from the best grade.

Number of students with Grade A = $15$
Number of students with Grade B = $27$
Total students with Grade A or B = $15 + 27 = 42$
Since exactly 42 students made it through, the cutoff is Grade B.