If the cost is C, then the markup is .6C and so the retail price is 1.6C. I assume that there are additional cost (store rental, salaries, etc.) so that minus .25C, which is (1.6- .25)C= 1.35C, is equal to the Total cost. Now the problem asks for a markdown, m, so that m(1.6C)= 1.35C. That is, of course, m= 1.35/1.6., written as a percent.