I'm checking answer for my sister. But since I no longer have the background in how to appropriately use the financial formulas, I post it here for help.

Problem: A car that costs $10,000. I want to buy this car.

-I have $2,500 to deposit right now toward a down payment. I can get a 6.5% interest rate if I deposit this $2,500 into a CD. This is a one time deposit of $2,500 compounded monthly and it is locked in for 2 years.

-When I purchase this car in 2 years, I want to make $100 monthly payments for 3 years, and I project this loan will be at 4.2% interest.

Question: What do I need to deposit monthly into an annuity for the next 24 months to obtain the rest of the down payment? I can earn 5% interest compounded monthly with this annuity.

My first attempt I got $149.97, but my sister's answer is different ($145.85).

I got confused after the $100 monthly payments part; and switched between different equations for PV and FV, thus different answers. There's a play of words here and it confuses me. I don't know if the rest of the down payment after 2-year-CD will have the 4.2% interest or the $10,000.