54. If a contract provides for payments other than those mentioned in section 53, the credit rate computed in accordance with the Act is the credit rate that, where utilized according to the computation method prescribed in section 52, produces amounts whose sum is equal to the aggregate of the credit charges indicated in the contract on the condition that:(a) the parties perform their obligations, in the manner prescribed in the contract; and that
(b) the duration of each period is that prescribed in the contract.