QUESTION: My bank and savings and loan are both on the government's list of troubled institutions. I have less than $100,000 on deposit in each but more than $100,000 when the two are combined. If both were to fail, would each cover my deposits to $100,000? Or would the government regulators add the two amounts together and insure the sum to $100,000 only?--W. J. H.

ANSWER: Don't lose any sleep. Bank and S&L; deposit insurance is calculated separately by each institution. So, as long as you spread your money around and never keep more than $100,000 on deposit in a single bank or savings and loan, you're fully covered.

(Because banks and savings and loans calculate the maximum deposit insurance on the basis of what they call banking relationships, there are ways to keep several hundred thousand dollars in a single financial institution and still be fully insured. Say you are married and you and your spouse both have separate accounts, as well as a joint account, all in a single bank. Those are regarded as three separate banking relationships, and each would be insured to $100,000. Individual retirement accounts also are entitled to separate insurance, again up to $100,000.)

Don't, however, mistake separate branches of the same institution for different institutions. Some savers mistakenly think they can get around the insurance ceiling by divvying up their money among several offices of the same bank or S&L.; That won't work. For insurance purposes, there is no difference between having your money in 10 Bank X branches in 10 different cities or having it all in a single Bank X branch.

Q: In last week's column, you compared yields on a $10,000 investment for three different compounding periods when the interest rate remains constant--10%. Since you advised us readers to figure yields for ourselves, I took out my calculator and retraced your steps. The monthly and yearly compounding figures come out right. But the only way I was able to arrive at your answer for the interest earned in one year on a $10,000 investment at 10% interest compounded daily--$1,067--was to do the equivalent of mixing metaphors: Using both a 365-day year and a 360-day year in the same formula. Can that be right? And is that the way it is actually done in the business-financial-banking world?--F. S.

A: Of all the many readers who did the calculation themselves and were confused enough to write in, you were the only one to figure out how we arrived at interest of $1,067, or a yield of 10.67%.

Although it does indeed conjure up visions of mixed metaphors, this is one of many legitimate ways to calculate daily-compounded interest. And if your institution happens to be one that figures it that way, you should feel fortunate. It means more money in your pocket. In this case, consistency--using only a 360-day year or a 365-day year--would earn you less.

Since several readers asked for the compounding formula so they can calculate yields themselves instead of relying either on a calculator or someone at a financial institution, here it is: Yield = P((1 + r / n)(to the * n* th power) - 1), where P is the principal; r is the annual interest rate and n is the number of compounding periods.

The troubling question arises with "n." Should you use 365 days or 360 days as the number of compounding periods? There's no one right answer. Some financial institutions use 365 days as the number of compounding periods, while others use 360. And some use both, as we did last week. To get the daily interest rate, we divided 10% by 360 days. And for the number of compounding periods, we used 365.

If you plugged either 365 or 360 into both "n" spots, you would have arrived at a number slightly less than $1,052, or 10.52%.

Q: I'm interested in contributing retirement money to a spousal IRA. I know the maximum yearly contribution is $2,250. But does that mean I have to contribute $2,000 to my own IRA and $250 to my wife's?--I. G.

A: No more than $2,000 of the $2,250 yearly contribution may be deposited into either account. But otherwise, you may divide it up any way you wish.

To qualify for a so-called spousal IRA, one spouse must be working and the other must not be. The one who isn't being compensated must be younger than 70 1/2. The couple must be legally married as of Dec. 31 of the tax year for which the deduction is claimed. And they must file a joint tax return.

If you meet all the rules, you and your wife simply open two separate IRAs and make your contributions.

* Debra Whitefield cannot answer mail individually but will respond in this column to financial questions of general interest. Do not telephone. Write to Money Talk, Business Section, The Times, Times Mirror Square, Los Angeles 90053. *