$6-12.$ Two pumps are being considered for a certain service of $10$ years. The Toltec pump has a first cost of $$8,000$ and an estimated annual cost of $$3,200$ for electric energy used for pumping purposes. The Mandan pump has a first cost of $$6,400.$ Because it is less efficient, its annual costs for pumping energy will be $$340$ higher than those of the Toltec pump$/$ Estimated annual income taxes will be $$32$ higher with the Toltec pump. It is anticipated that there will be no other cost differences an no salvage values. What is the prospective after$-$tax rate of return on the extra investment required to purchase the Toltec pump?