The contract made no mention of the downward floating exchange rate. When the engineering quantity exceeds 15%, the comprehensive unit price of the increased engineering quantity should not be reduced. According to Article 9.6.2 of the Code for Valuation with Bill of Quantities of Construction Projects (GB 50500-2013+03), if the deviation of quantities specified in this section and the engineering changes specified in Section 9.3 lead to the deviation of quantities exceeding 15%, adjustments can be made. When the engineering quantity increases by more than 15%, the comprehensive unit price of the increased engineering quantity should be lowered; When the quantity of work is reduced by more than 15%, the comprehensive unit price of the remaining quantity after reduction shall be increased.