The balance after 21 months is $9960.
Let a₁ = 14760, a₂ = 14520, and a₂₁ be the balance after 21 months:
So we apply the given values to get the missing values:
First, we get the common difference, That is,
d = a₂ - a₁
d = 14520 - 14760
d = -240
So, we substitute the value of common difference and the known values,
a₂₁ = a₁ + (n-1)d
a₂₁ = 14760 + (21-1)(-240)
a₂₁ = 14760 + (20)(-240)
a₂₁ = 14760+(-4800)
a₂₁ = 9960
Therefore, the balance after 21 months is $9960.
