Write an equation (in dollars) for the total interest earned in terms of x,y, and z. Find a general description for the amounts invested (in dollars) at the three rates. (Express your answer in terms of z.) (x,y,z)=() (b) If $12,000 is invested at 10%, how much should be invested at each of the other rates? 8%$ 7%$ What if $28,000 is invested at 10% ? 8%$ 7%$ (c) What is the minimum amount that should be invested at 7% ? $1 In this case, how much should be invested at the other rates? 8%10%$$ (d) What is the maximum amount that should be invested at 7% ? $ In this case. how much shoyld be invested at the other rates? 8% $ 10%