Q: My wife and I have $15,000 that we have been placing in six-month savings certificates. The return has been around 10 percent. Since real estate seems to be one of the very best investments, would you suggest that we buy an acre of land or a lot rather than the savings certificates? Or is there another form of investment we should look into? We are in our late 30s.

A: If you buy land, be certain it's well-located, readily usable and can be developed and sold. It should be likely to appreciate in value at a rate in excess of 10 percent a year, plus your carrying costs, if any (which may be up to 15 percent a year or more). That sort of land may be hard to find. It requires knowledge and experience to be able to recognize it. It provides little or no income, in all probability.

So, income-producing real estate is a better investment for you, I think. If you can get together enough money for a down payment on a sound, well-located single-family house, that may be your best real estate investment. You'll want the rental income at least to be sufficient to meet your mortgage payments and other annual expenses, so you won't have a negative cash flow.

It's even better, of course, if you can get a positive cash flow. If your property is sound and well-located, in our present economic climate, it should increase in value 12 to 15 percent or more a year.

If you can't find such a single-family house, investigate investing in a real estate syndicate.

Q: I'm a real estate agent. I listed for sale a duplex owned by three persons as joint tenants. The joint tenant who signed the listing agreement said he represented the other joint tenants and they agreed to sell at the price shown in the listing agreement. Now I have a buyer ready, willing and able to buy at the listed price. But one of the joint tenants says the listing joint tenant didn't represent him. He refuses to sell. Am I entitled to the sales commission?

A: According to the weight of authority, you're entitled to your agreed-upon sales commission. The listing joint tenant is liable for that sum of money. The other two joint tenants are not liable, the courts have said.

Q: My family and I moved to a new location because of a job change. We couldn't sell our home because lending institutions in our area shut off all mortgages. We rented our home and took a second mortgage to buy a home temporarily. We've discovered that our former home, which is now rented, may be in a location that is zoned commercial. Now we want to buy the home we were unable to buy 11 months ago. What are our tax obligations?

A: Even the Internal Revenue Service isn't in complete agreement on your tax obligations. But it appears to me that you can defer payment of tax on any gain on the sale of your former principal residence, providing you sell it while you're still living in your present (temporary) principal residence.

If it is more than 18 months after the sale of your former principal residence, you can defer payment of tax on any gain on its sale. If it's within 18 months of the sale, you can defer tax only on the gain on your present (temporary) home.

This assumes that (1) the cost of your new home in each instance equals or exceeds the adjusted sales price of your former home, and (2) the purchase of each is within 18 months after the sale of the former house.

If the cost of a new house is less than the adjusted sales price of a former home, the gain taxed is the lesser of the gain on the sale of a former principal residence, or the excess of the adjusted sales price of a former home over a new home. The tax on any remaining gain will be deferred until a later year, unless you're ultimately able to take advantage of the $100,000, age 55-and-over, one-time tax exclusion on the sale of your home.

I base this on Internal Revenue Service Revenue Ruling 59-72 and information contained in Internal Revenue Service Publication 17, Your Federal Income Tax, 1979 edition, pages 110-116, and particularly page 12. For details, see your advisor or the nearest Internal Revenue Taxpayer Service Office. It may also be wise to attempt to get a ruling on your tax obligations from the Internal Revenue Service.