DEAR BOB: I think I've solved an old problem of landlords and tenants that you may wish to pass along. When an apartment tenant moves out, there is often disagreement with the landlord about whether all, part, or none of the security deposit should be refunded. I've solved that problem by eliminating security deposits. If, for example, an apartment rents for $200 per month and I formerly charged a $150 security deposit, I now charge $350 for the first month's rent and $200 per month thereafter. I find the tenants leave the apartments in about the same condition as before and I've eliminated all hassles over security deposits. Henry M., Washington.

DEAR HENRY: Sounds like a great idea. I think I'll give it a try.

DEAR BOB: I own a store building. Three years ago the tenant, with my permission, installed a new store front and put in a fancy new ceiling. His lease expires next month. He says if he moves out, I owe him the cost of these improvements (about $16,000). I gave him permission to make the improvements, but I never agreed to reimburse him for the cost. Must I pay? Susan E., Bethesda.

DEAR SUSAN: It appears your tenant is trying to bluff you. The general rule is that, in the absence of any contrary agreement between the landlord and tenant, the tenant's improvements to the structure become the landlord's property upon expiration of the lease.

In the meantime, the tenant amortizes the cost, over the remaining term of his lease. Consult your attorney, of course, to see if any circumstances make this general rule inapplicable to your situation.

DEAR BOB: We're not sure if we correctly filed our income tax reture reporting the sale of our home in 1977. My husband didn't take any deduction for the settlement attorney or the settlement fees. We sold one home and bought another. Ethel L., Silver Spring.

DEAR ETHEL: Your settlement costs are a sales expense that is subtracted from your old home's gross sales price to arrive at the "adjusted sales price." Then subtract your home's "adjusted cost basis" from the "adjusted sales price" and the result is your profit.

Since you bought another home, if you used the "residence replacement rule" to defer paying your profit tax, your settlement costs reduced the profit on which the tax is deferred. It would be wise to have your tax adviser or attorney look over your tax returns to be sure you didn't cheat yourselves out of some present or future tax savings.

DEAR BOB: Is joint tenancy with right of survivorship limited to just two joint owners? We're buying a home and want to put title, if possible, in joint tenancy with myself, my wife, and our son. Tommy A., Vienna.

DEAR TOMMY: There is no limit to the number of joint tenants who can own a property. But before acquiring title, talk it over with your attorney to be certain joint tenancy is really the best way to hold title.

The report "How Senior Citizens Can Save Taxes When Selling Their Residence" is available for 25 cents plus a stamped self-addressed envelope sent to Robert J. Bruss, P.O. Box 6710, San Francisco, Calif. 94101.