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21. a² = c² - b²
= (255 cm)² - (225 cm)²
= 65.025 cm² - 50.625 cm²
= 14.400 cm²
√(a²) = √(14.400 cm²)
a = 120 cm

22. Rata-rata = Jumlah data/ Banyak data
= (3 + 4 + 5 + 5 + 5 + 6 + 6 + 8 + 8 + 9)/ 10
= 59/ 10
= 5,9

23. Rata-rata = Jumlah data/ Banyak data
80 = (60 + 70 + 90 + 95 + n)/ 5
80 × 5 = 315 + n
400 - 315 = n
n = 85

24. Rata-rata = Jumlah data/ Banyak data
6 = (6 + x + 7 + 2x - 4)/ 4
6 = (9 + 3x)/ 4
6 × 4 = 9 + 3x
24 - 9 = 3x
15 = 3x
x = 15 ÷ 3
x = 5
Jadi urutan datanya : 5, 6, 6, 7
J = x maksimal - x minimal = 7 - 5 = 2

25. Penyelesaian :

20 × 48,6 kg = 16 × 48,5 kg + 4 × n
972 kg = 776 kg + 4n
4n = 972 kg - 776 kg
= 196 kg
n = 196 kg ÷ 4
= 49 kg