For Question 1, assuming that the initial deposit is made on 1/1 of the year, since bank compounding for quarterlies occurs on 3/31, 6/30, 9/30, 12/31 of each year, you want:

=2579.05

The 7.6% per annum is 1.9%/quarter. 18 full years with 4 quarters per year is 72 quarters

This can be verified running this to check the accumulated balance like a 401(k) that is rolling here -> Balance Roll with Interest

Say it is 1/1/2008 > 1/1/2026 when the balance is collected. Roll up 2579.05 at 7.6% compounded quarterly, which is 1.9% per quarter for 18 years with all 72 credited quarterlies and you will get your 10000.

Question 2:

Compound interest takes the previous balance from the last compounding period. So we do this as follows:

7.2% per annum = 7.2/12 = .6% = .006 per month.

Balance after 3 months =

Total Interest Earned = Accumulated Balance - Principal = 4072.43 - 4000 = 72.43