The audit of this item varies according to the nature of the account and the manner of keeping it. Unless a form which is similar to Form 15 (Section 23) is used, the profits in a Gain on Sales account, including profits from all time sales, whether on mortgage or contract, excluding subdivision sales, can best be kept and verified by a plan such as is described in Section 304. This method applies also to those sales where interest and other charges are from time to time debited to the Income account, and the amounts paid by purchasers do not therefore represent the reduction of the indebtedness, as a portion of such payments is first applicable to interest and expenses. The profit earned in these cases depends not on the amounts paid in, but on the amount by which the debt is reduced.