Take a look at the attachment to see the solution.
A = future value
P = principal (P = 12,000)
r = interest rate (r=6)
n = time periods (n=12)
I'm terrible at math, so I'm sorry if I'm totally wrong, but I'm assuming you just take the average:
20+28=48m
48/2=24m
On average, it may take them 24m if they work together.
You know that d = 100 and w = 125, so you plug that into the equation and solve as shown:
b= 100 + d - w
= 100 + 100 - 125
= 200 - 125
= $75
You now have $75 left as balance.
2.3+0.23 = 2.53
2.53+0.023 = 2.553