Financial mathematics increasing annuities I am having trouble solving this problem

Julie bought a house with a 100,000 mortgage for 30 years being repaid with payments at the end of each month at an interest rate of 8% compounded monthly.  If Julie pays an extra 100 each month, what is the outstanding balance at the end of 10 years immediately after the 120th payment?

My attempt:
I first want to find the deposit per month.
I let $D$ be the deposit per month and since it increases by 100 each payment, I used an increasing annuity,
$D*100(Ia_{30|0.08}) = 100,000$
However, the $D$ I got was 8.12, which is clearly not right.
Can someone help?

HINT:
I have no formulas, but good old spreadsheet works wonders:
I calculated the monthly principal to be paid:
$277.7777
Then the sum of the all the interest compounded monthly over the thirty years:
$120,333.3333
TOTAL principal+interest=$220,333.3333
So amount to pay per month is $612.037 during 360 months.
If you decide to instead pay $712.037 during 120 months,
you will have outstanding balance at month 121 of $120,603.7037
(The decimals are there to show data is not rounded to cents).
Depending on the bank or country practices... I forgot to estimate the outstanding balance if those $100 go directly to principal: you will finish to pay in 317 months, thus reducing both the time and interest.
Outstanding Principal just after month 120 (no interest)=$62,145.11041
