Wappo, a company that manufactures turbine blades for hydroelectric dams, has designed a new model of blade with a superior hydrodynamic form that will save energy and hence cost in implementation. Wappo is willing to sell these blades to a particular hydroelectric dam at no initial cost. The only payment will be the difference between, first, a percentage of the dam's improved profits, as measured by the percentage improvement of the hydroelectric dam efficiency times the value of energy sold by the plant over an 18-month period, and, second, the additional cost of installation and maintenance of the new turbines. On installation, the dam will make an estimated payment, which will be adjusted after eighteen months to equal the proper amount.