Tolong dijawab pake cara please..

jadi nilai A nya adalah 30

[tex] \frac{(60 \times 40) + (70 \times 20) + (80 \times a) + (90 \times 20) + (100 \times 15)}{40 + 20 + a + 20 + 15} [/tex]
[tex] \frac{2400 + 1400 + 80a + 1800 + 1500}{95 + a} = 76[/tex]
[tex] \frac{7100 + 80a}{95 + a} = 76[/tex]
7100+80a = 76 x (95 + a)
7100+80a = 7220+76a
80a-76a = 7220-7100
4a=120
a=30