tlong jwb dan pembahasan skarang

deret aritmatika
90 - 85 - 80 - ...
a = 90
b = 85 - 90 = -5

Sn = n/2 (2a + (n-1)b)
S12 = 12/2 (2(90) + (12-1)(-5))
= 12/2 (180 + (-55))
= 6 (125)
= 750 kg