The agreement shall provide for a down payment in the amount of twenty percent of the total purchase price of the cost of the lot, and shall further provide for the payment of monthly installments over a period not to exceed twenty months. The monthly installments shall be in the amounts equal to the balance of the contract divided by the number of months which the contract is to run, plus two months extra payment to pay for the privilege of making the payments in installments or six percent of the balance, whichever is less. (Code 1975 § 8-261(B))