[tex]\begin{aligned} \sf Rata-rata &= \sf \frac{Jumlah \: semua \: skor }{Jumlah \: frekuensi} \\ \sf 7&= \sf \frac{(5 \times 20) + (6 \times 20) + (7 \times 20) + (8 \times a) + (9 \times 10) }{(20 + 20 + 20 + a + 10)} \\ \sf 7&= \sf \frac{100 + 120 + 140 + 8a + 90}{(70 + a)} \\ \sf 7&= \sf \frac{450 + 8a}{(70 + a)} \\ \sf 7(70 + a)&= \sf 450 + 8a\\ \sf 490+7a&= \sf 450+8a \\ \sf 490-450 &= \sf 8a-7a \\ \sf 40 &= \sf a\end{aligned}[/tex]
Jadi, nilai a adalah 40.
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[tex]\begin{aligned} \sf Rata-rata &= \sf \frac{Jumlah \: semua \: skor }{Jumlah \: frekuensi} \\ \sf 7&= \sf \frac{(5 \times 20) + (6 \times 20) + (7 \times 20) + (8 \times a) + (9 \times 10) }{(20 + 20 + 20 + a + 10)} \\ \sf 7&= \sf \frac{100 + 120 + 140 + 8a + 90}{(70 + a)} \\ \sf 7&= \sf \frac{450 + 8a}{(70 + a)} \\ \sf 7(70 + a)&= \sf 450 + 8a\\ \sf 490+7a&= \sf 450+8a \\ \sf 490-450 &= \sf 8a-7a \\ \sf 40 &= \sf a\end{aligned}[/tex]
Jadi, nilai a adalah 40.