740 - 707.6 = 32.4
707.6 - 675.20 = 32.4
675.20 - 642.8 = 32.4
Balance = 740.0 - 32.4 (n-1), where n - 1 is the humber of months elapsed.
The second question may be answered by finding when the sum of the terms equal the original amount in the account.
This is: Sum of S1 + S2 + S3 + ... = 740.0
Now you can either use the equation that gives the sum of terms of an arithmetic progression- which is Sn = [A1 + An]*n/2, where A1 is the first term and An is the nth term, or going down 32.40 step-by-step.
The result is that at month 23 the account will have 27.20, then you have to deposit to pay the next months.